Aid workers are helping refugees from violence in Darfur
A man in military fatigues shot the 37-year-old Spanish woman, a Unicef employee, in the town of Abeche and made off with her jeep, witnesses said.
The woman, who was wounded in the arm, was taken to the capital, N'Djamena, where officials said she was in a serious condition.
Aid workers are in the eastern part of Chad to assist refugees from the conflict in Sudan's Darfur region.
About a quarter of the relief personnel in eastern Chad pulled out last month over security fears following a rebel attack in the capital.
In recent months, rebels and robbers have been taking advantage of a security vacuum in the region after the army withdrew to key towns.
Aid officials said attacks on humanitarian workers were increasing as the security situation deteriorated. More than 20 vehicles belonging to relief agencies have been stolen, Reuters news agency said.
"We really need the authorities to ensure our security."
About 200,000 refugees have crossed the border into Chad to escape the violence in Darfur.
The Government of Sudan has expressed that the  Darfur Peace Accord with main rebel group, Sudan Liberation Army, led by Minni Menawi, can overturn the previous rejection of UN Peacekeepers. So far, only African Union Peacekeepers were allowed in Darfur.

"The Sudan government will be open for any assistance," Bakri Mulah, secretary-general for external affairs in Sudan's Information Ministry, said in Khartoum, Sudan.

In New York, John R. Bolton, the US ambassador, on Saturday welcomed Khartoum's new willingness to accept a UN peacekeeping force to take over from AU soldiers in the Darfur region. He cited the Sudanese government representatives' comments  to indicate they were now willing to accept the UN force. He said, " We view this as a very encouraging sign, the first positive outcome from the Abuja peace agreement. "

In the meantime, top UN humanitarian official Jan Egeland has arrived in Sudan to review the situation in the Darfur region. He is expected to visit southern Darfur, where fighting has broken out recently.  It is the first visit by a UN official to the region since the peace deal was signed.

He said that access for aid workers in Darfur was at its worst level in two years. "At the moment Darfur is slowly being strangled, it's dying in front of us, half of the population now has become war victims...so I believe, yes, we are turning the corner, but the whole world has to put pressure on the parties." said Mr Egeland, the UN's Emergency Relief Co-ordinator.  

Earlier, UN Secretary General Kofi Annan requested Khartoum to issue visas to his team of assessors so they could plan for the deployment of an international peacekeeping force to replace the 7,000 African Union troops later in the year. The AU troops, now running out of funds, had largely been ineffective in stopping atrocities, leaving millions to struggle in camps with little food or water. While details for a UN peacekeeping force are finalised, the US diplomat said the US had asked Rwanda to send in some 1,200 troops to supplement the AU forces.

In a recent development, an unknown assailant shot a Spanish United Nations Aid worker in Chad. She is working in the eastern part of Chad to assist the refugees from Darfur region. 2,00,000 refugees have crossed the border to escape from the violence in Darfur. She is in critical condition.