The ruling junta said it it did not know if pro-democracy leader Aung San Suu Kyi was refusing to take food and her own supporters also said they could not confirm the report.
According to US officials, the Nobel laureate is protesting her illegal detention by the military government.
"We are deeply concerned for her safety and well-being," State Department deputy spokesman Philip Reeker said without saying how the US heard that she had launched a hunger strike.
Myanmar did not specifically deny the US hunger strike charge but the government said it was "confused" by it.
"We firmly believe it quite odd for the United States State Department to make such a claim without stating any sources to
verify its allegation," the government said in a statement.
A senior member of Suu Kyi's National League for Democracy
(NLD) could not confirm the US assertion.
"We have learned this news from the international media and
we are really concerned about her health, but we still cannot
confirm whether it is true or not," he told Reuters.
The State department's comments came only two days after a tough new raft of US economic sanctions came into force, aimed at pressuring the junta over Aung San Suu Kyi's detention.
"The Burmese authorities have full responsibility for her health, having placed her in so-called ‘protective custody’ since a May 30 premeditated attack on her convoy," he said.
Reeker also called for Suu Kyi's immediate release.
The US statement came just a day after Myanmar's new prime minister, General Khin Nyunt, announced a seven-point roadmap towards democracy including "free and fair" elections to be held under a new constitution.
But no timeframe was given for the elections and observers in Yangon have already dismissed the speech as a rehash of previous promises to restore a tightly restricted form of democracy to Myanmar.
"The Burmese authorities have full responsibility for her health"
US State Department
The junta has promised a gradual transition to democracy in the past, but there has been little progress as the bitter dispute with the opposition continues.
Aung San Suu Kyi, born in 1945, is revered as an icon of democracy in the West, but is barely tolerated by the ruling generals in Myanmar.
She returned to Myanmar from a comfortable family life in Britain in April 1988 to nurse her sick mother, but found Yangon in the midst of protests against the military.
Thousands left dead
Any aspirations for democracy quickly vanished as demonstrations turned into a bloodbath, leaving thousands of protestors dead.
Demonstrations spread nationwide and grew in intensity, prompting her to take on the role of intermediary in what was a bloody confrontation.
In September 1988 she helped found the NLD, an alliance of opposition parties challenging the junta, and began travelling around Myanmar to deliver more than 100 campaign speeches for the NLD, ahead of national elections.
She led the NLD to a landslide victory in 1990, despite being confined to her home. But the generals refused to accept the results.
Over the next decade, the military mounted a campaign of intimidation, repression and arrests directed at the NLD. But by 2000 the NLD was devastated and left with little but the fame of its leader.
After periods in and out of house arrest, Suu Kyi was freed last year, with a promise from the junta that she would be able to travel freely, but her liberty was short-lived, ending earlier this year.