in relief and reconstruction aid, U.S. Ambassador Ryan Crocker said.
"The magnitude of this disaster is utterly overwhelming," Crocker said. "We have under way the beginning of a very major relief effort."
The United Nations said more than 2.5 million people were left homeless by Saturday's monster 7.6-magnitude quake, and doctors warned of an outbreak of disease unless more relief arrives soon.
With landslides blocking roads to many of the worst-hit areas, Pakistan's army was flying food, water and medicine into the disaster zone. International relief efforts cranked into action, and an American plane full of relief supplies landed at an air base near Pakistan's capital on Monday.
Most of the dead were in Pakistan's mountainous north. India reported more than 800 deaths; Afghanistan reported four.
In the shattered streets of Muzaffarabad, where at least 11,000 people died, an Associated Press reporter saw shopkeepers scuffle with people trying to break into businesses. They beat each other with sticks and threw stones, and some people suffered head wounds. No police were nearby.
Residents of Muzaffarabad, the capital of Pakistan's portion of divided Kashmir, said looters also targeted deserted homes. Survivors lacked food and water, and there was little sign of any official coordination of relief in the devastated city of 600,000.
An eight-member team of British rescuers using a sniffer dog, drills, chain saws and crowbars pulled a 20-year-old tailor from the rubble on Monday afternoon, 54 hours after a two-story building collapsed over him and dozens of others.
The man, Tariq, was wide-eyed and covered in dust when he emerged, and he begged for water.
"I haven't eaten in three days, but I'm not hungry," said Tariq, who suffered a leg injury and was carried away on a door serving as a stretcher. He had been trapped beneath concrete and wooden beams, and a dead body lay on either side of him.
About 2,000 people huddled around campfires through the cold night on a soccer field on the city's university campus, where most buildings had collapsed and hundreds were feared buried in classrooms and dormitories. Soldiers burrowed into the concrete with shovels and iron bars.
President Gen. Pervez Musharraf said the earthquake was the country's worst on record and appealed for urgent help, particularly cargo helicopters to reach remote areas.
President Bush on Sunday promised cash and said he had told Musharraf "we want to help in any way we can."
U.S. forces in Afghanistan prepared to send five Chinook transport helicopters and three Blackhawk helicopters to Pakistan on Monday to help ferry relief supplies.
"Pakistan is one of our closest allies in the war on terror and we want to help them in this time of crisis," said Sgt. Marina Evans, a U.S. military spokeswoman in Kabul. "The terrorists make us out as the infidels, but this is not true, and we hope this mission will show that."
India, a longtime rival of Pakistan, offered help in a gesture of cooperation. The nuclear-armed neighbors have been pursuing peace after fighting three wars since independence from British rule in 1947, two of them over the Kashmir region.
Other international aid, including emergency rescue workers, began to flow in. Planes arrived from Turkey, Britain, Japan and the United Arab Emirates. Russia, China and Germany also offered assistance.
Pakistani Prime Minister Shaukat Aziz said his country's death toll was 19,396 and was expected to rise.
Senior officials in Pakistan's portion of Kashmir put the death toll much higher. The top elected official in the region, Sardar Sikandar Hayat, said that more than 25,000 people had died there with "countless" injured. Tariq Mahmood, the province's communications minister, put the toll at over 30,000.
Troops "have not started relief work in remote villages where people are still buried in the rubble, and in some areas nobody is present to organize funerals for the dead," Mahmood said.
The quake was felt across a wide swath of South Asia, with damage spanning at least 250 miles, from Jalalabad in Afghanistan to Srinagar in northern Indian territory.
In Geneva, the United Nations urgently appealed for donations, including for at least 200,000 winterized tents.
On the Indian side of the militarized Kashmir border --- where at least 800 have died --- hundreds of Kashmiris spent Sunday night outside in the cold after rumors of another temblor. Hundreds of mosques announced warnings of a further quake over loudspeakers; none was reported.