Condoleezza Rice, the US secretary of state, urged Myanmar's generals on Wednesday to admit international disaster relief, saying it was a humanitarian crisis rather than a political issue.
"What remains is for the Burmese government to allow the international community to help its people," Rice told reporters in Washington.
"It should be a simple matter. It's not a matter of politics. It's a matter of a humanitarian crisis," Rice said.
Five days after the cyclone hit the country, most people have still not received survival items.
Shari Villarosa, the charge d'affaires of the US embassy in Myanmar, said: "The information that we're receiving indicates that there may well be over 100,000 deaths in the delta area."
Villarosa was speaking to reporters on Wednesday via conference call from the former capital, Yangon.
She said the situation in Yangon is "increasingly horrendous," citing relief agency reports of shortages of food and drinking water.
John Holmes, the UN humanitarian chief, also said on Wednesday that the country was facing a "major catastrophe", and urged the military government to facilitate the arrival of disaster relief teams and the distribution of supplies.
The military, which has ruled the country for more than 40 years and recently faced a popular uprising over food and fuel prices, has been slow to issue visas to aid workers and is accused of hampering relief work through numerous travel restrictions.
State Myanmar radio and television, the main official sources for casualties and damage, reported an updated death toll of 22,980 with 42,119 people missing and 1,383 injured in one of Asia's deadliest cyclones.
Most of the victims were swept away by a wall of water when the cyclone hit coastal towns and villages in the rice-growing delta southwest of Yangon.
Despite having two days warning before the cyclone struck, the government failed to give its people adequate warning.
Richard Horsey from the United Nations Office of the Coordination of Humanitarian Affairs (OCHA), said: "We estimate upwards of one million people currently in need of shelter and life-saving assistance."
He added that 5,000 square km of the delta is under water.
Aid began entering Myanmar on Wednesday, five days after the cyclone hit.
However, the international community is concerned over whether the country's ruling military government will open up to a full-scale international relief operation.
The US and Australia have appealed to the government to accept assistance from Thailand, China, India and Indonesia which were flying in relief supplies.
France has called for the UN Security Council to get involved and ordered two of its navy vessels to prepare to deliver humanitarian aid.
Holmes, speaking at a news conference in New York, said four Asian members of a UN disaster assessment team who do not need visas had received clearance to enter Myanmar on Thursday.
A fifth non-Asian member is still waiting.
Holmes, who called on Myanmar to waive visa requirements for aid workers, said a World Food Programme (WFP) plane is expected to arrive in Myanmar early on Thursday.
The UN's Office for the Co-ordination of Humanitarian Affairs said another flight with aid and staff will leave at the end of the week from southern Italy. The UN's refugee agency also said 22 tonnes of emergency supplies were waiting in trucks at the border with Thailand waiting for permission to enter.
OCHA said the WFP had already been able to distribute some food aid in Yangon, and aid has also arrived from Thailand and China.