The junta's new laws have been widely criticised internationally
The US says Burma's newly-enacted elections laws are a "setback" for political dialogue and the junta's engagement with Washington.
The comments from Assistant Secretary of State Kurt Campbell came as Burma's military rulers unveiled the last of five widely-criticised election laws.
No date has yet been set for the poll, the first in Burma for 20 years.
Meanwhile, a UN special envoy to Burma has accused the country of a "gross and systematic violation of human rights".
In a leaked report, Tomas Ojea Quintana called for an investigation into potential war crimes in Burma.
Burma's military leaders say they plan to hold multi-party elections this year. They have been announcing the laws under which the polls will be held all week.
Many key opposition figures - including detained leader Aung San Suu Kyi - are not allowed to participate.
Members of religious orders are also banned from taking part, a rule which covers the monks who led anti-government protests two years ago.
Speaking on a visit to Thailand, Mr Campbell said the US was "very disappointed and ... concerned" by the new laws.
"It's very regrettable. This is not what we had hoped for, and it is a setback," he said.
Last year, the US said it would seek to engage directly with Burma's rulers in a bid to promote democratic ideas, saying sanctions alone would not be effective.
Mr Campbell said the approach had been to "try to encourage domestic dialogue between the key stakeholders" but that the enactment of the new laws "doesn't leave much room for such a dialogue".
He said Washington would continue to talk to leader Gen Than Shwe and the junta, but that sanctions would remain in place.
His comments came a day after Mr Quintana's report, which contained severe criticism of the Burmese authorities.
Mr Quintana expressed concern about the treatment of minorities in Burma
Mr Quintana said there was a "pattern of gross and systematic violation of human rights" in the country.
"The possibility exists that some of these human rights violations may entail categories of crimes against humanity or war crimes," he said, in the report to be presented to the UN on Monday.
Mr Quintana, who has visited Burma on three occasions, said "far too many" people in the country were denied access to basic food, shelter, health and education.
He also highlighted concerns about ongoing abuses of civilians in conflict areas around Burma's borders - including the recruitment of child soldiers - and about the treatment of Muslim ethnic groups in the north.