Typhoon Haiyan, one of the strongest storms ever recorded, struck the Philippines in November 2013 with winds of up to 190 mph (305 kph). At least 10,000 people died in one Philippine province alone.
Philippines Typhoon Haiyan death toll exceeds 10,000 as rescue effort continues
Rescue workers overwhelmed by sheer scale of devastation after Haiyan wreaks havoc in what could be the Philippines' worst natural disaster
Agence France-Presse in Tacloban
As many as 10,000 people are believed to have died in one Philippine province alone as one of the worst storms on record ravaged several central islands, burying people under tonnes of debris and leaving corpses hanging from trees.
The horrifying estimates - which would make it the country's worst recorded natural disaster - came as rescue workers appeared overwhelmed in their efforts to help countless survivors of Super Typhoon Haiyan.
Hundreds of police and soldiers were deployed to contain looters in Tacloban, the devastated provincial capital of Leyte, while the United States announced it had responded to a Philippine government appeal and would send military help.
Video: More than 10,000 feared dead in typhoon-ravaged Philippines
"There's an awful lot of casualties, a lot of people dead all over the place, a lot of destruction," Richard Gordon, head of the Philippine Red Cross, told the BBC.
"It's absolute bedlam right now, but hopefully it will turn out better as more and more supplies get into the area."
Authorities were struggling to even understand the sheer magnitude of the disaster, let alone react to it, with the regional police chief for Leyte saying 10,000 people were believed to have died in that province alone.
"We had a meeting last night with the governor and, based on the government's estimates, initially there are 10,000 casualties [dead]," Chief Superintendent Elmer Soria said in Tacloban. "About 70 to 80 per cent of the houses and structures along the typhoon's path were destroyed."
The destruction extends well beyond Tacloban. Officials had yet to make contact with Guiuan, a town of 40,000 that was hit first by the typhoon. Baco, a city of 35,000 people in Oriental Mindoro province, was 80 per cent under water, the UN said.
On the island of Samar, a local disaster chief said 300 people were killed in the small town of Basey. He added another 2,000 were missing there and elsewhere on Samar, which was one of the first areas to be hit when Haiyan swept in from the Pacific Ocean with maximum sustained winds of 315km/h.
Haiyan moved out of the Philippines and into the South China Sea on Saturday, from where it tracked towards Vietnam. Although it weakened out at sea, more than 600,000 people were evacuated in Vietnam ahead of its expected landfall this morning.
China was also bracing for the approaching storm. Some 250 flights were delayed or cancelled at the international airport in the Hainan tourist hub of Sanya . A nearby highway was also shut down. Guangdong-Hainan train-ferry services were suspended.
About 16,000 passengers were left stranded, mainland media reported.
Xinhua reported that six sailors were missing from a Guangxi cargo ship after it was blown out to sea from a Sanya harbour. According to the online news portal of Haikou , Hainan province was hit by strong winds and heavy rain.
Video: Super Typhoon Haiyan makes landfall in the Philippines
Additional reporting by Reuters, Mimi Lau