More than 10,000 people were killed in a devastating cyclone that hit western Burma on Saturday, Foreign Minister Nyan Win has said on state TV.
He said his government was ready to accept international assistance. Aid shipments are now being prepared.
Thousands of survivors of Cyclone Nargis are lacking shelter, drinking water, power and communications.
The United States offered to increase aid offered if Burma agreed to allow a US team access to assess the situation.
First Lady Laura Bush, who takes a special interest in Burma, urged Burma to accept $250,000 (£126,000) already allocated for emergency aid, and said more would be available if the team was allowed into the country.
She also accused the Burmese authorities of failing to give a "timely warning" about the approaching storm, after which five regions - home to 24 million people - have been declared disaster zones.
Expressing his sadness at the scale of the disaster, UN Secretary General Ban Ki-moon confirmed that UN officials were meeting Burmese government representatives to discuss how to help.
If the toll is confirmed, Nargis is now the world's deadliest storm since a 1999 cyclone in India killed 10,000 people.
Nargis hit the south-east Asian country on Saturday with wind speeds reaching 190km/h (120mph). It brought with it a sea surge that smashed through towns and villages.
But damage to roads and communications mean it is impossible to tell the true extent of the situation, he added.
Prices of food, fuel and basic necessities have also risen dramatically.
The UN and international aid agencies are sending assessment teams to the worst-hit areas and shipments are being prepared.
Thailand has announced it is flying in a transport plane loaded with nine tonnes of food and medicines and India is sending two naval ships carrying food, tents, blankets, clothing and medicines.
The US, which released an immediate funding package of $250,000 (£127,000) to be channelled through the UN, said that Burmese authorities had refused permission for an American disaster assistance response team to enter.
Meanwhile, Burma's military junta has said a referendum on a new national constitution will go ahead on Saturday. People were "eagerly looking forward to voting", it said.
But some people are now wondering if this natural disaster could have serious political repercussions, reports the BBC's Andrew Harding in the Thai capital Bangkok.
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