U.S. Defense Secretary Chuck Hagel directed the Pacific Command to deploy ships and aircraft to support search-and-rescue operations and fly in emergency supplies.
U.N. Secretary-General Ban Ki-moon was "extremely concerned" by the widespread destruction and the steeply rising death toll, according to a statement released by his office.
Ban said the U.N. and its humanitarian partners "have quickly ramped up critical relief operations" even though many communities remain difficult to reach, the statement added.
Pope Francis led tens of thousands of people at the Vatican in prayer for the victims. The Philippines has the largest number of Catholics in Asia, and Filipinos are one of Rome's biggest immigrant communities.
The Philippines is annually buffeted by tropical storms and typhoons, which are called hurricanes and cyclones elsewhere. The nation is in the northwestern Pacific, right in the path of the world's No. 1 typhoon generator, according to meteorologists. The archipelago's exposed eastern seaboard often bears the brunt.
Even by the standards of the Philippines, however, Haiyan is a catastrophe of epic proportions and has shocked the impoverished and densely populated nation of 96 million people. Its winds were among the strongest ever recorded, and it appears to have killed more people than the previous deadliest Philippine storm, Thelma, in which about 5,100 people died in the central Philippines in 1991.
The country's deadliest disaster on record was the 1976 magnitude-7.9 earthquake that triggered a tsunami in the Moro Gulf in the southern Philippines, killing 5,791 people.
Defense Secretary Voltaire Gazmin said Aquino was "speechless" when he told him of the devastation in Tacloban.
"I told him all systems are down," Gazmin said. "There is no power, no water, nothing. People are desperate. They're looting."
Tacloban, in the east-central Philippines, is near the Red Beach on Leyte Island where U.S. Gen. Douglas MacArthur waded ashore in 1944 during World War II and fulfilled his famous pledge: "I shall return."
It was the first city liberated from the Japanese by U.S. and Filipino forces and served as the Philippines' temporary capital for several months. It is also the hometown of former Filipino first lady Imelda Marcos, whose nephew, Alfred Romualdez, is the city's mayor.
One Tacloban resident said he and others took refuge inside a Jeep, but the vehicle was picked up by a surging wall of water.
"The water was as high as a coconut tree," said 44-year-old Sandy Torotoro, a bicycle taxi driver who lives near the airport with his wife and 8-year-old daughter. "I got out of the Jeep and I was swept away by the rampaging water with logs, trees and our house, which was ripped off from its mooring.
"When we were being swept by the water, many people were floating and raising their hands and yelling for help. But what can we do? We also needed to be helped," Torotoro said.