Russia and the United States have agreed "all the documents" for a successor to a landmark nuclear disarmament treaty and will likely sign it in Prague, a Kremlin source told AFP on Wednesday.

"As of now, all the documents on the new START treaty have been agreed upon," said the Kremlin source, who spoke on condition he not be named, suggesting an apparent breakthrough in the long-stalled talks.

The comments indicated that Moscow and Washington were finally set to sign a successor pact to the landmark 1991 Strategic Arms Reduction Treaty, a key goal of the administration of US President Barack Obama.

Obama has made talks on replacing START the central element of his efforts to "reset" strained US-Russian relations, but they had reportedly been bogged down by disagreements over US missile defence.

Earlier on Wednesday, a US official, speaking in Washington on condition of anonymity, said the White House had spoken with Russia and the Czech Republic about its desire to sign the new treaty with Russia in Prague, essentially designating the Czech capital as the signing venue for the new treaty.

The Kremlin source confirmed that Prague was the "most likely" site for the signing.

"Prague is indeed considered the most likely venue for the signing of the new START treaty," he said.

Moscow's ambassador to the Czech Republic confirmed Wednesday that the treaty would be signed in Prague.

The Czech capital carries special symbolism for Obama, who in a major speech in Prague last April called for a world free of nuclear weapons.

Moscow and Washington have held months of difficult negotiations aimed at replacing the treaty, a cornerstone of Cold War-era strategic arms control which expired in December.

Signed in 1991, START led to huge reductions in the Russian and US nuclear arsenals and imposed verification measures to build trust between the two former Cold War foes.

Delays in the START talks and missed deadlines have cast a shadow over the Russian and US leaders' efforts to make good on their pledge to improve bilateral ties.

The United States is set to host a nuclear security summit on April 12-13, and observers have said it is a matter of pride for Washington to have the new treaty in place before the summit.

The Czech TV network TV Nova, citing Russian diplomatic sources, said on its web site that the signing would take place on April 8, adding that an official announcement would be made to that effect in Moscow at 2000 GMT Wednesday.

Mikhail Margelov, the chairman of the foreign affairs committee of the upper house of Russia's parliament, confirmed Wednesday that the text of the new nuclear agreement was ready.

"The text is ready, we are starting to think how to conduct the ratification," he said on Echo of Moscow radio.

"We are preparing procedures related to the simultaneous ratification of this treaty which should take place after it is signed by the presidents of Russia and the United States."

Margelov expressed hope the treaty would pass ratification fairly quickly in Russia, while adding it may encounter difficulties in the US Senate.

To facilitate ratification in the two countries, a Russian parliamentary group would head to the United States for meetings with US Senators in "mid-April, virtually right after the nuclear summit", he said.

He suggested the new treaty would contain a link between missile defence and reductions in strategic offensive weapons -- a position which Moscow has insisted upon.

"A political compromise has been reached," Margelov said.