The United States and Russia have made "significant" progress towards a new nuclear disarmament treaty, President Dmitry Medvedev was quoted as saying on Saturday.

"We have taken a fairly serious step forward, and to a significant degree we have brought our positions closer," Medvedev said, quoted by the ITAR-TASS and RIA-Novosti state news agencies.

The negotiations "are not proceeding easily, but on many positions we have reached agreement with the Americans," the president said during a meeting with senior Russian lawmakers.

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 President Barack Obama and Medvedev have pledged to replace START, but the talks have got bogged down in recent months amid disgreements over measures for missile monitoring.

Medvedev also warned that once a new agreement was reached, US and Russian lawmakers should ratify it simultaneously, saying it would be "unacceptable" for Russia's Duma to ratify the treaty while the US Senate rejected it.

"Either we move together in ratifying a treaty which is well-reasoned and reflects our plans for the future of strategic nuclear forces, or else this process will not happen," Medvedev said.

The US constitution requires a two-thirds majority vote in the Senate to ratify any treaty, and the Senate is traditionally less predictable in its voting than the Duma, which usually follows the will of the Kremlin.

Some senators, mostly Republicans, have threatened to block ratification of the START successor treaty if Obama makes too many concessions on nuclear disarmament to Moscow.

On Thursday, a top US diplomat, US Under Secretary of State William Burns, said the United States and Russia were "on the verge" of completing the new agreement after months of closed-door talks.

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, signed in the closing days of the Cold War, 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 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.