Belarus said Friday that a new round of US sanctions has forced it to suspend plans to eliminate highly enriched uranium under an agreement reached with Secretary of State Hillary Clinton last year.

The announcement represents a stark escalation in the two sides' standoff over the former Soviet republic's human rights record following President Alexander Lukashenko's unprecedented crackdown on the opposition.

It was not immediately clear how much of the dangerous material Belarus actually possessed or what it intended to do with the amounts it had.

The Belarussian foreign ministry said the latest sanctions imposed against four state companies by Washington last week "were politically motivated and in contradiction of the United States' international obligations."

"Under these circumstances, Belarus has decided to freeze the projects being developed jointly with the United States on exchanging highly enriched uranium," it said in a statement.

The nuclear agreement freeze in effect represented the most serious response Belarus could have taken against a country with which it has almost no formal trade ties.

December's deal came as a coup for Clinton and offered the first signs of a thaw between two nations that have had strained relations for most of the past two decades.

Clinton at the time called the agreement "a sign of progress in efforts to advance nuclear security and non-proliferation."

But that upbeat assessment was followed weeks later by a controversial presidential election in which veteran Belarus leader Lukashenko secured victory in a poll that led to mass street protests and arrests.

The subsequent human rights crackdown broke a cautious rapprochement in Minsk's relations with the European Union and prompted Washington to impose several rounds of economic and other penalties against Lukashenko's team.

The latest round imposed last week targeted four strategic manufacturers in the Belneftekhim conglomerate and prompted an immediate vow of reprisals from Minsk.

The idea that Belarus -- which agreed to destroy its post-Soviet nuclear stockpiles in the 1990s -- had dangerous fissile material that could be made into a weapon first emerged in 2008.

Lukashenko that year unexpectedly mentioned that his country had highly enriched uranium without specifying its source or purpose.

Some members of the opposition at the time suggested that Lukashenko was simply bluffing in an attempt to secure more aide from the Unites States.

But the head of the Sosny nuclear research institute outside the capital Minsk said Friday that his institute was in fact enriching uranium to a higher level under a programme whose details he did not reveal.

Vyacheslav Kuvshinov said his facility had already shipped 10 percent of its highly enriched uranium stock to Russia under the US programme announced by Clinton.

"The next stage of this project will be halted in line with the foreign ministry decision," the institute's director told the Interfax news agency.

He added that the stock on site would be kept safe and out of the hands of "foreign persons or organisations".

December's deal saw Washington agree to provide the technical and financial assistance required to help Belarus eliminate the dangerous stockpiles by the summer of 2012.