Senior officials from the United States and Russia held talks Thursday on a new nuclear disarmament treaty, Interfax news agency reported, after both sides said significant progress had been made.

US and Russian negotiators have been in lengthy talks on a replacement for the 1991 Strategic Arms Reduction Treaty (START), a key Cold War-era disarmament pact which expired in December.

US National Security Adviser James Jones and Chairman of the Joint Chiefs of Staff Mike Mullen took part in the discussions in Moscow with their Russian counterparts, Interfax said without providing the names of the Russian officials.

"The discussions are occurring behind closed doors," Interfax quoted a Russian diplomatic source as saying.

Russian President Dmitry Medvedev said last weekend that the two countries had made "significant" progress towards a new nuclear disarmament treaty.

US Under Secretary of State William Burns said recently that the two sides were "on the verge" of a deal.

The broad outlines of the new treaty have been clear since a summit in July, when US President Barack Obama and Medvedev agreed that the replacement treaty should slash the number of warheads on either side to between 1,500 and 1,675.

The presidents also agreed that the number of carriers capable of delivering the warheads should be limited to between 500 and 1,100.

The United States has said it currently has some 2,200 nuclear warheads, while Russia is believed to have about 3,000.