The so-called 123 agreement, which lays the groundwork for closer cooperation on nuclear research, production and trade, effectively entered into force on December 9.

"The agreement opens opportunities for Russia-U.S. cooperation in third countries and lifts restrictions on Russian supplies of highly enriched uranium," said Mikhail Margelov, head of the foreign relations committee in the upper house of the Russian parliament.

"Under the agreement, U.S. companies will be able to buy [nuclear] fuel in Russia without any restrictions."

The 123 agreement was submitted to Congress by former President George W. Bush but recalled following Russia's armed conflict with Georgia in August 2008.

It was resubmitted by President Barack Obama in May in a bid to "reset" relations with Russia.

Critics have argued the deal rewards Russia despite its ties with Iran where it has been building a nuclear reactor.

Russia's Foreign Ministry has said the pact will foster cooperation in research on the handling of spent nuclear fuel, the provision of services related to the nuclear fuel cycle, and the development of technology to improve nuclear security.