Around 10,000 people may have died in just one area of the Philippines hit by Typhoon Haiyan, according to officials.
One of the worst storms on record, it destroyed homes, schools and an airport in the eastern city of Tacloban.
Neighbouring Samar island was also badly affected, with reports of 300 people dead and 2,000 missing.
The Philippine government has so far only confirmed the deaths of 151 people throughout the country, but hundreds of thousands have been displaced.
The BBC's Rupert Wingfield-Hayes reports that the scene in Tacloban, the capital of Leyte province, is one of utter devastation.
Houses in Tacloban have been flattened by the massive storm surge that accompanied Typhoon Haiyan.
There's no clean water, no electricity and very little food.
City officials said they were struggling to distribute aid and that looting was widespread.
In some areas, the dead are being buried in mass graves.
Our correspondent says hundreds of people are at the airport, itself badly damaged, trying to get on a flight out of Tacloban.
The typhoon is now bearing down on Vietnam. More than 600,000 people have been evacuated in northern provinces.
At least four people were reported killed there, apparently while trying to escape the storm.
The BBC Weather Centre says the typhoon is expected to make landfall south of Hanoi on Monday afternoon local time (between 03:00 and 09:00 GMT), although it will have decreased markedly in strength.
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Via Chuck Sherwood, Senior Associate, TeleDimensions, Inc, London Hines