Mongolia, jammed between nuclear arms giants Russia and China, on Monday secured a unique pledge from the major powers not to breach its self-declared nuclear weapons-free zone.

The UN ambassadors for Britain, China, France, Russia and the United States -- the five permanent members of the UN Security Council and original nuclear weapons powers -- signed a declaration after marathon talks.

Mongolia declared itself free of nuclear weapons 20 years ago but, because it is landlocked between Russia and China, could not join a regional bloc free of weapons.

The major UN powers in turn did not want to recognize a one-country nuclear weapons free zone so complex talks were held on the statement signed Monday.

The UN powers signed a declaration promising "to respect the nuclear weapon free status of Mongolia and not to contribute to any act that would violate it," Britain's UN ambassador Mark Lyall Grant said.

Jargalsaikhan Enkhsaikhan, Mongolia's ambassador to the UN atomic energy agency in Vienna, said his country wanted to be part of efforts to halt the spread of nuclear weapons. "It does not want its territory to be a vast undefined grey area in this respect," he told reporters.

He said Mongolia would have preferred a legally binding treaty but the declaration was "tantamount to recognition of Mongolia's neutrality in nuclear powers' possible power politics or designs."