A senior US diplomat has spoken of his "profound disappointment" at Burma's preparations for an election this year.
The Assistant Secretary of State for East Asia warned the ruling junta that the poll would not be recognised by the international community.
Kurt Campbell met Burma's detained democracy leader Aung San Suu Kyi on Monday amid concern over planned elections in the military-run nation.
Mr Campbell also called for the release of all political prisoners in Burma.
'Take immediate steps'
Speaking in the city of Rangoon on Monday, Mr Campbell said the regime had chosen to "move ahead unilaterally".
He said: "As a result, what we have seen to date leads us to believe that these elections will lack international legitimacy.
"We urge the regime to take immediate steps to open the process in the time remaining before the elections," he said.
The senior envoy met Ms Suu Kyi at a state guesthouse in Rangoon during a five-day visit to the country.
Ms Suu Kyi's party was forcibly disbanded last week, under the new election laws. Her party, the National League for Democracy, won the last elections in 1990 but was not allowed to take power.
Ms Suu Kyi has spent most of the last two decades in some form of detention and is currently under house arrest in Rangoon.
Mr Campbell did not reveal details of his talks with Ms Suu Kyi.
But he said: "It is simply tragic that Burma's generals have rebuffed her countless appeals to work together to find a peaceable solution for a more prosperous future."
The envoy also delivered a strong warning over Burma's arms deals, saying the regime should abide by UN sanctions that prohibit buying arms from North Korea.
Some analysts suspect these deals includes nuclear technology.
Burma's generals have not yet set a date for what they say will be multi-party elections later this year.
But their newly-enacted election laws ban anyone with a criminal conviction from playing any role in the polls - forcing the NLD to choose between expelling Ms Suu Kyi and other top leaders, or disbanding.
Last week the deadline for the NLD to comply with the election laws and register for the polls passed. The party is now officially defunct, though a faction within the movement says it will form a new political party.
Before meeting Ms Suu Kyi, Mr Campbell met other NLD leaders, who called for a stronger line from Washington on Burma.
"We also told Mr Campbell not to recognise the results of the upcoming election, which will be held without the two important elements - credibility and inclusivity - that the international community has demanded," top NLD leader Win Tin said.
This is Mr Campbell's second visit to Burma - he also met Ms Suu Kyi in November 2009.
That visit followed the announcement by the US of a new policy of pragmatic engagement with the military junta.
But the BBC's South East Asia correspondent, Rachel Harvey, says that there are few expectations of major progress resulting from this trip.