However, the prime minister also defended British diplomats, insisting that some had been working "round the clock" and had only managed to enter New Orleans itself overnight.
Mr Blair spoke after the first British survivors began arriving home yesterday with horror stories about the New Orleans Superdome stadium, the city's main emergency shelter.
Speaking at Gatwick airport, Christine Robertson, whose daughter, Cora, 22, was in the stadium for four days, said yesterday: "They left these very young people in a foreign country in fear of their lives. They just abandoned them. They did nothing to support them."
Some spoke of intimidation and harassment by men at the stadium and other witnesses have told of rapes, murders and suicides.
Mr Blair said he understood the anger but said the disaster had created a "confusing and difficult" situation, which had been "more shocking and serious than people contemplated".
Mr Blair, speaking in a round of broadcast interviews in Beijing where he was holding trade talks, said he was "really sorry if there has been difficulties".
He said: "It's been really tough for people, I know that, but it's been tough for our officials on the ground."
In Washington yesterday, the deputy British ambassador, Alan Charlton, said officials had previously been denied access to New Orleans and were doing everything they could.
The Bush administration has been criticised for its sluggish early response to the disaster.
"Overnight we have got people there in New Orleans for the first time, actually people to be on the spot to handle this," Mr Blair said.
The prime minister said he expected the current total of around 130 missing Britons to fall, and added that the UK was helping to coordinate a European aid package, including ration packs and camp beds to be sent to the stricken areas.
New Orleans was "effectively going to be rebuilt", he said.
At RAF Brize Norton in Oxfordshire this morning, 500,000 military ration packs worth £3m were loaded on to civilian charter planes to be flown to Little Rock in Arkansas.
From there, the supplies will be flown on to troops and survivors evacuated from the disaster area who are now in Houston, Texas, and in less affected areas of Mississippi and Alabama.
A further 15 aircraft are expected to fly out later this week and the operation could be extended to provide blankets, tents and cooking equipment.
British Prime Minister Tony Blair has apologised to Britons affected by Hurricane Katrina. Many British citizens who were trapped in the New Orleans have complained to the British Foreign Office about the lack of response from their government after the hurricane destabilized almost 200 miles of the United States gulf coast.

British nationals living and vacationing in New Orleans reported that U.S. authorities had refused to evacuate them as Hurricane Katrina approached the city, leaving them stranded in their hotels.  President George W. Bush has been criticized for refusing material aid from France and Britain in rescuing people trapped in New Orleans.