US Senator Jim Webb has cancelled a visit to Burma because of concern the government might be working with North Korea to develop a nuclear programme.
Citing reports containing "new allegations" of possible co-operation, Mr Webb said it would be "unwise" for him to visit Burma for now.
It was not immediately clear which reports Mr Webb was referring to.
Last year he went to Burma to help free a US man who made an uninvited visit to pro-democracy leader Aung San Suu Kyi.
Mr Webb said it was not clear whether the new allegations had "substantive merit".
"Until there is further clarification on these matters, I believe it would be unwise and potentially counter-productive for me to visit Burma," he said.
US Assistant Secretary of State for East Asia Kurt Campbell warned Burma last month to abide by UN sanctions that prohibit buying arms from North Korea.
Some analysts suspect these deals include nuclear technology.
During his visit last year, Mr Webb met the reclusive military leader, General Than Shwe, winning the release of American John Yettaw.
The Burmese government normally shuns outsiders and accuses them of interference.
He also met Ms Suu Kyi, who has been under house arrest for 14 of the past 20 years.
Washington has recently changed its approach to Burma, adopting a policy of deeper engagement.