The head of the Red Cross in the Philippines has described the devastation caused by Typhoon Haiyan as "absolute bedlam".
Officials estimate up to 10,000 people have died in Taclo
ban city and hundreds elsewhere. Hundreds of thousands of people are displaced.
The typhoon flattened homes, schools and an airport.
It has since made landfall in northern Vietnam, near the Chinese border, where it has weakened to a tropical storm.
Four million people have been affected in the Philippines, and many are now struggling to survive without food, shelter or clean drinking water.
A huge international relief effort is under way, but rescue workers have struggled to reach some towns and villages cut off since the storm.
"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."
He said roads had now been cleared to allow relief workers to get to the hardest hit areas, but that they expected to find many more casualties.
"It's only now that they were able to get in and we're beginning just to bring in the necessary food items... as well as water and other things that they need."
Forecasters predicted a tropical depression would move into the south and central Philippines on Tuesday, potentially bringing heavy rains that would further hamper relief efforts.
Jane Cocking, the humanitarian director for Oxfam, said her colleagues witnessed "complete devastation... entire parts of the coastline just disappeared, and sizable trees just bent over and [were] thrown about like matchsticks."
The latest report from the Philippines' Disaster Risk Reduction and Management Council confirmed 255 deaths as of 22:00 GMT on Sunday. It said almost 630,000 people had been reported displaced.
But officials have warned that the number of dead will rise significantly.
Typhoon Haiyan - one of the most powerful storms on record to make landfall - swept through six central Philippine islands on Friday.
It brought sustained winds of 235km/h (147mph), with gusts of 275 km/h (170 mph), with waves as high as 15m (45ft), bringing up to 400mm (15.75 inches) of rain in places.
"The world has not seen a storm like this before," said Senen Mangalile, the Philippines Consul General to the UK.
Steven Godby, a disaster management expert at Nottingham Trent University, told the BBC the typhoon was "probably the most intense and strongest storm of this type to make landfall".
"We've seen storms like this perhaps on rare occasions that have had that kind of intensity out at sea but for it to come ashore with that kind of strength is almost unprecedented," Dr Godby said.
American military aircraft and ships are being deployed to provide help. US President Barack Obama has issued a message saying he was "deeply saddened by the loss of life and extensive damage".
Other countries have also pledged millions of dollars in assistance. Australia has approved $9m in humanitarian aid to the Philippines, while New Zealand has pledged over $1m.
Kristalina Georgieva the EU humanitarian aid commissioner, said relief efforts would be guided by three priorities.
"The first one is to get access to remote areas as quickly as possible, and the access issue is both transportation and also restoring telecommunications," she said
"The second, of course, is to get the immediate humanitarian assistance for people affected by this kind of disaster. And the next one would be shelter."
The relief efforts are being focused on the eastern province of Leyte and its capital Tacloban, where a massive storm surge flattened houses.
The typhoon - named "Yolanda" by Filipino authorities - wrecked up to 80% of structures in its path, police chief superintendent Elmer Soria told reporters.
Officials said looting was widespread and order was proving difficult to enforce.
Military spokesman Lt Col Ramon Zagala said that 100 soldiers had been deployed to help police prevent looting in Tacloban, AFP news agency reported.
Philippine President Benigno Aquino said there was a possibility that martial law or a state of emergency would be declared in the city.
In some areas, the dead are being buried in mass graves.
Typhoon Haiyan has now made landfall in Vietnam, near the tourist destination of Ha Long Bay, with sustained winds of up to 140 km/h (85mph).
Some 600,000 people were evacuated in northern provinces of the country.
Source: (Story) BBC News
