In a statement late on Sunday the US state department said it had learned that Ms Suu Kyi had stopped eating "to protest her illegal detention by [the] military regime". It gave few other details, but said Washington was "deeply concerned for her safety and well-being".
The junta, known officially as the state peace and development council, poured scorn on the claim. The foreign ministry said the report was "groundless". "We believe it is quite odd for the United States to make such a claim without stating any sources to verify its allegation," it added.
But no Burmese officials were willing to comment on Ms Suu Kyi's welfare. She is being held in "protective custody", according to the junta, which claims she is at risk of assassination. She has been kept in the notorious Insein jail in the capital, Rangoon, but her current whereabouts is unknown.
Her party, the National League for Democracy, was unable to confirm the hunger strike claim. Many of its senior leadership are in jail, detained with Ms Suu Kyi on May 30 after her party convoy was attacked in northern Burma, allegedly by several thousand soldiers and regime thugs.
It is thought that dozens of her supporters were killed in the ambush near Mandalay and about 100 were detained. Many NLD offices have since been closed down.
Representatives of the International Committee for the Red Cross in Rangoon, who were the last foreigners to see the Nobel peace laureate, on July 28, were also unable to shed light on the situation.
Alfredo Mallet, the deputy ICRC representative, said: "We will know about it when we see her again." But no date has been set for such a meeting.
The British Foreign Office minister Mike O'Brien said yesterday he was "deeply concerned" by the US report.
"I told [the Burmese ambassador to Britain] that the only way we could be sure if the reports were true would be if the International Committee of the Red Cross could visit Aung San Suu Kyi as soon as possible," Mr O'Brien said. "The British government holds the Burmese authorities responsible for Aung San Suu Kyi's health and welfare."
The head of the Burma UK Campaign, John Jackson, urged Britain, which has just taken over the chair of the UN security council, to put Burma on the council's agenda.
"If Aung San Suu Kyi is on hunger strike it is a last resort, as she has been abandoned by the UN," Mr Jackson said. "Her arrest and the massacre of up to 100 of her supporters has only been discussed for a few minutes under 'any other business'. It's a disgrace."
The human rights group Amnesty International believes there are about 1,400 political prisoners in Burma, some of whom have been detained for decades.
If the US government report is correct, analysts believe Ms Suu Kyi's move could be linked to the junta's announcement on Saturday that it was resurrecting its "road map" to democracy. This was shelved in 1996 after the NLD, which won Burma's last general election in 1990 but was never allowed to take office, walked out on the talks after refusing to consent to giving the military a formal role in politics.
When announcing the road map, the junta suggested there would be no role for Ms Suu Kyi or the NLD in the new national convention, which will write a new constitution and pave the way for "free and fair elections". No date for the transition to democracy was given.
Two of Burma's neighbours, China and Thailand, warmly welcomed the initiative by the junta's new prime minister, General Khin Nyunt.
Gen Khin Nyunt took over the previously ceremonial position last week in a reshuffle of the government, which saw several ministers "retire".
It is not clear whether this is a promotion or demotion for the regime's former third-ranked official, who is still thought to be head of the powerful military intelligence bureau, because the junta has become more secretive than ever.
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