(CNN) -- Aid organizations have deployed emergency response teams to Haiti and appealed for donations after the Caribbean nation was was struck by a devastating earthquake described by local officials as a "catastrophe of major proportions."
The magnitude 7.0 earthquake struck southern Haiti on Tuesday, knocking down buildings and inflicting a new catastrophe on the western hemisphere's poorest nation.
Humanitarian charity Oxfam said Wednesday it was rushing rescue teams to the country from around the region to provide clean water, sanitation, shelter and emergency supplies and called for donations to fund its efforts.
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"At this stage it is too early to tell the severity of the earthquake in Haiti, but the early signs are not good with communications down across the country," said Jane Cocking, humanitarian director of Oxfam.
Kristie van de Wetering, a former Oxfam employee based in the Haitian capital Port-au-Prince, said the situation in the capital was "very chaotic" with many buildings reduced to rubble.
"We can hear people calling for help from every corner. The aftershocks are ongoing and making people very nervous," she said.
The International Federation of Red Cross and Red Crescent Societies said volunteers in Haiti were assisting the injured and supporting hospitals which had been overwhelmed by the disaster.
It said it had enough supplies in Haiti for 3,000 families. Experts in disaster response are due to arrive in the country later Wednesday to coordinate international relief efforts, it said.
"The most urgent needs at this time are search and rescue, field hospitals, emergency health, water purification, emergency shelter, logistics and telecommunications," the group said in a statement.
The quake struck about 15 km (10 miles) southwest of Port-au-Prince shortly before 5 p.m. local time, cutting off communications across much of the country.
"Port-au-Prince is devastated, lot of deaths. SOS. SOS...," wrote Louise Ivers, the clinical director of medical charity Partners In Health, in an e-mail to the group's offices in Boston, Massachusetts.
She added: "Temporary field hospital ... needs supplies, pain meds, bandages. Please help us."
Raymond Joseph, Haiti's ambassador to the U.S., told CNN's Wolf Blitzer by telephone that the country was going throug
"I'm calling on all friends of Haiti and people who are listening to me to please come to our aid," said Raymond Joseph, Haiti's ambassador to the U.S. told CNN's Wolf Blitzer by telephone.
"Today as Haiti is going through the worst day in its history I am calling for all others who got help from us in the beginning to help in support," Joseph said. "The only thing I can do now is pray and hope for the best."
Singer Wyclef Jean, nephew of ambassador Joseph, stressed the need for help for what is considered among the poorest nations in the Western Hemisphere.
"We're going to need immediate aid," Jean told Blitzer on CNN. "We're going to need the United States and the international community to react immediately." He founded Yele Haiti, whose community service programs include food distribution and emergency relief.
In Washington U.S. President Barack Obama said the government would "stand ready to assist the people of Haiti."
At the Pentagon, the U.S. military said humanitarian aid was being prepared for shipping, but it was not yet clear where or how it would be sent. A U.S. aviation source said the control tower at the Port-au-Prince international airport collapsed, possibly hindering efforts to fly relief supplies into the country.
Secretary of State Hillary Clinton told reporters that Washington is offering "our full assistance" to Haiti. "And our prayers are with the people who have suffered, their families and their loved ones," she said. The deputy chief of the U.S. mission in Haiti, David Lindwall, told Clinton that he saw "significant damage" from the quake and said U.S. officials there expect "serious loss of life," Crowley said.
And Clinton's husband, former U.S. President Bill Clinton -- now the U.N. special envoy for Haiti -- said the world body was "committed to do whatever we can to assist the people of Haiti in their relief, rebuilding and recovery efforts."
Haiti's government is backed by a U.N. peacekeeping mission established after the ouster of former President Jean-Bertrand Aristide in 2004.
The United States has been heavily involved in Haiti commercially, politically and militarily for most of the last century. U.S. intervention under Clinton restored Aristide to power in 1994 after a 1991 coup, and a U.S. jet hustled him out of the country again in 2004 following a rapidly spreading uprising against his government.
With people stripping the trees for fuel and to clear land for agriculture, the mountainous countryside has been heavily deforested. That has led to severe erosion and left Haitians vulnerable to massive landslides when heavy rains fall.
Roads in Haiti were unsafe to travel on because of a lack of lighting and because many buildings along transportation routes had collapsed or were not deemed safe, said Ian Rodgers of the relief organization Save the Children.
"What I can hear is very distressed people," Rogers said. "There is a lot of distress and wailing of people trying to find loved ones."
A representative for the aid group Catholic Relief Services in Haiti described the situation in the nation as "a total disaster," said Robyn Fieser, regional information officer for the group.
Haiti's dense population will increase the risk to its people, Jean said. The nation's need for aid will range from water and food to medical and building supplies.
"This is the worst devastation that we as Haitian people have faced," he said.
Hurricane Gordon killed more than 1,000 people in 1994, while Hurricane Georges killed more than 400 and destroyed the majority of the country's crops in 1998. And in 2004, Hurricane Jeanne killed more than 3,000 people even as it passed north of Haiti, with most of the deaths in the northwestern city of Gonaives.
Gonaives was hit heavily again in 2008 when four tropical systems passed through.
According to the U.N. Office for the Special Envoy for Haiti, unemployment reaches 70 percent nationally, and 78 percent of Haitians live on less than $2 a day.
CNN's Edvige Jean-Francois, Shasta Darlington, Deb Feyerick, Matt Smith, Mike Mount and Pierre Meilhan contributed to this report.
Map of where the quake struck.
A 7.0 magnitude earthquake has struck off the coast of Haiti earlier today at 21:53 UTC, according to the US Geological Survey (USGS). According to the US National Oceanic and Atmospheric Administration, no tsunami warning was issued, contradicting some media reports that said there was one in place. The quake's magnitude was revised down from an initial report of 7.3 on the Richter scale.

"A destructive widespread tsunami threat does not exist based on historical earthquake and tsunami data," the US Pacific Tsunami Warning Center said. "However, there is the possibility of a local tsunami that could affect coasts located usually no more than a 100 km 60 miles from the earthquake epicentre."

The USGS reports that the epicentre was fifteen kilometres (ten miles) southwest of Port-au-Prince and  southeast of Miami, Florida, at a depth of ten kilometres.

According to USGS analyst Dale Grant, this tremor was "the largest quake recorded in this area"; the last strong quake was in 1984, and had a magnitude of 6.7, he noted.

"Everybody is just totally, totally freaked out and shaken. The sky is just gray with dust," said Henry Bahn, an official with the US Department of Agriculture who was visiting in Haiti and a witness to the incident. "I just held on and bounced across the wall. I just hear a tremendous amount of noise and shouting and screaming in the distance," he commented, as quoted by the Associated Press.

A map indicating the intensity of the earthquake in Haiti and the Dominican Republic, made by USGSAccording to data from the USGS, there were thirteen aftershocks after the initial tremors, the strongest of which was 5.9 on the Richter scale; seven others measured in at 5.0 or higher – all the aftershocks were 4.0 or greater.

No deaths have yet been reported, but a hospital in Port-au-Prince was damaged, and a US government official said several houses fell into a ravine. An analyst for USGS in Colorado commented that there could be many deaths from an earthquake of this strength. "I think we are going to see substantial damage and casualties," he said.

The USGS's Mike Blanpied remarked that about three million people were impacted by the earthquake, given its location. "This quake occurred under land as opposed to off-shore, so a lot of people were directly exposed to the shaking coming off that earthquake fault, which was quite shallow," he said.

A lack of communications makes it difficult to ascertain what damage was done. US embassy personnel were "literally in the dark" due to power failures, said State Department spokesman P. J. Crowley. "They reported structures down. They reported a lot of walls down. They did see a number of bodies in the street and on the sidewalk that had been hit by debris. So clearly, there's going to be serious loss of life in this," he said, as quoted by AP. A reporter for Reuters said he had seen "dozens of dead and injured people" amidst collapsed buildings.

"We are trying to get in touch with our people on the ground but we are experiencing communication problems, which is not unusual in a disaster such as this," said United Nations spokeswoman Stephanie Bunker to Agence France-Presse.


=== Aid appeals and international reaction ===

Appeals for aid flooded in following the disaster. In a phone call to CNN, the Haitian ambassador to the US, Raymond Joseph, said: "I'm calling on all friends of Haiti and people who are listening to me to please come to our aid. Today as Haiti is going through the worst day in its history I am calling for all others who got help from us in the beginning to help in support. The only thing I can do now is pray and hope for the best."

US Secretary of State Hillary Clinton promised that her country would also provide assistance to Haitians. "The United States is offering our full assistance to Haiti and to others in the region. We will be providing both civilian and military disaster relief and humanitarian assistance and our prayers are with the people who have suffered, their families and their loved ones."

President Barack Obama also said that the country would be "ready to assist" Haiti.