A Russian plan to build a nuclear research centre in Burma has been heavily criticised by US officials.
Burma's military government is subject to international sanctions
State Department spokesman Tom Casey said the Asian state had neither the "legal frame" nor the "safeguard provisions" for a nuclear programme.
Russia's atomic energy body, Rosatom, said the project would be monitored by the UN's nuclear agency, the IAEA.
The US and EU maintain sanctions against Burma, accusing its military rulers of human rights abuses.
Rosatom announced the deal in Moscow on Tuesday, saying the centre would include a 10-megawatt light-water reactor and facilities for processing and storing nuclear waste.
In a statement, Russia pledged to train 300-350 specialists for the centre - although no timescale for construction was given.
Mr Casey said the US had a "general sense" that Burma had "neither the regulatory or legal frame nor safeguard provisions" for a country to be able to handle such a programme.
He expressed concern that there were "no accounting mechanisms or other kinds of security procedures" to prevent nuclear fuel from being stolen.
"We would be concerned about the possibility for accidents, for environmental damage, or for proliferation simply by the possibility of fuel being diverted, stolen or otherwise removed," he said.
The proposed reactor could not be used for a nuclear weapons programme, says the BBC's Steven Eke.
But the deal will again raise questions about Russia's willingness to export nuclear know-how to countries the West considers repressive or hostile, our correspondent adds.
Russia's nuclear co-operation with Iran - who the US and other nations accuse of trying to develop nuclear weapons - has been a source of tension between Moscow and western nations.
The military have ruled Burma since 1962. Opposition leader Aung San Suu Kyi, whose NLD party won general elections in 1990, has been under house arrest in the country's former capital, Rangoon, for several years.