The United States and Russia are "on the verge" of a new nuclear disarmament treaty after months of talks, US Under Secretary of State William Burns said in comments released on Thursday.

"We're on the verge of completing the agreement and ... I'm optimistic that we'll complete it soon," Burns said, according to the transcript of an interview released by the US Embassy in Moscow as he was visiting Russia.

Washington and Moscow have been seeking to agree the terms of a new pact to replace the 1991 Strategic Arms Reduction Treaty (START), which expired on December 5.

US President Barack Obama and his Russian counterpart Dmitry Medvedev had originally sought to have a new treaty by December 5, but talks have gotten bogged down in recent months amid disagreements over various issues.

Many of the disagreements have centred on monitoring measures, such as US inspections of Russian missile facilities, which Moscow does not want to be included in the new treaty.

"There do remain a few issues, related mainly to verification, that have to be sorted through. But I believe they will be sorted through in the coming weeks," Burns said.

Burns added that a new US-Russian treaty would be "a very important signal to the rest of the world," especially ahead of a May review conference on the Nuclear Non-Proliferation Treaty.

Burns, who gave the interview to the Russian news website Gazeta.ru, did not elaborate on the results of his two-day visit to Moscow.

The Obama administration has made efforts to replace START a key element of its initiative to "reset" US-Russian relations that were badly strained during the previous administration of George W. Bush.

START, seen as a cornerstone of Cold War-era strategic arms control, imposed strict limits on the US and Russian nuclear arsenals and set up a system of mutual inspections and data exhcanges to ensure compliance.

The broad outlines of the new treaty have been clear since a summit in July, when 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 two 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.