A long-awaited nuclear cooperation agreement between Moscow and Washington allowing US companies to sell nuclear reactors and materials to Russia entered into force on Tuesday, officials said.

The cooperation agreement in the field of peaceful use of nuclear energy, also known as a 123 agreement, came into effect after US Ambassador to Russia John Beyrle and Russian Deputy Foreign Minister Sergei Ryabkov exchanged diplomatic notes in the Russian capital, a Russia foreign ministry spokesman told AFP.

The agreement strengthens nonproliferation efforts and allows US companies to sell nuclear materials, reactors and major reactor components to Russia.

The agreement will allow the two countries "to create new commercial opportunities for US and Russian companies to produce cleaner, safer, and more secure nuclear energy," Beyrle said in a statement released by the US embassy in Moscow.

Signed in 2008, the agreement was put on ice as bilateral ties deteriorated under ex-president George W. Bush. Barack Obama, who took over from Bush in 2009, made re-invigorating US ties with Moscow a major priority of his presidency.

Late last year Obama pushed through the ratification of a historic nuclear arms control treaty known as the new START, in a move hailed as a major political victory for the administration after a months-long battle.