Russia and the United States have agreed a landmark nuclear arms reduction treaty, the Kremlin said on Wednesday, but the White House cautioned some issues still needed to be worked out.

"All documents for the signing of START have been agreed," said the Kremlin official, who asked not to be identified. The official said Russian President Dmitry Medvedev and U.S. President Barack Obama would decide soon when to sign it.

In Washington, White House spokesman Robert Gibbs said "We are very close to having an agreement on a START treaty, but we won't have one until President Obama and his counterpart Mr. Medvedev have a chance to speak."

"There are still some things that need to be worked out," Gibbs told a daily news briefing.

Both sides said the successor to the last major Cold War arms reduction pact would likely be signed in Prague, capital of a former Soviet satellite that is now in NATO.

Russian and U.S. negotiators have been trying to hammer out a successor to the 1991 Strategic Arms Reduction Treaty (START I) for almost a year. They missed an initial deadline of Dec. 5, when START I expired.

The new pact is a crucial element of efforts to get Russian-U.S. relations on track after years of tension that peaked following Russia's brief war with U.S.-supported Georgia in 2008.

In a joint understanding last July, Obama and Medvedev said the treaty would reduce operationally deployed nuclear warheads to between 1,500 and 1,675 each, the specific number to be determined in talks.

Cold war treaty 

The most recent treaty to cut the Cold War foes' nuclear weapons numbers, signed by former presidents George W. Bush and Vladimir Putin before ties spiralled downward, limited each side to 2,200 warheads each by 2012.

U.S. and Russian officials say they hope further cuts in the world's largest nuclear arsenals would send a signal to other nations and help reduce the threat of armed conflict.

The START successor pact is a key goal for Obama as he seeks to rein in Iran's nuclear ambitions and promotes efforts to eventually rid the world of nuclear weapons.

The signing could take place around the anniversary of Obama's April 5, 2009, speech in Prague offering his vision for reducing global nuclear arsenals.

Analysts say the treaty is in Russia's interests, as well, estimating that its aging arsenal will drop below 1,500 warheads in less than a decade.

Signing a major arms treaty with the United States could help Russia bolster its image as a global power and improve relations with Washington amid persistent disputes on issues ranging from other weapons, to trade, to human rights.

But Russian officials -- most prominently the powerful Prime Minister Putin -- repeatedly cast doubt on the chances for a deal by suggesting that Moscow might not sign without U.S. concessions on the divisive issue of missile defence.

The Kremlin has expressed concern that further cutting its offensive arsenal without binding the United States to limits on defensive systems could upset the strategic balance in favour of Washington.

The July joint statement but said the treaty would include a provision on the relationship between offensive and defensive weapons. But as recently as this week, Russia's armed forces chief of staff suggested Moscow was not yet satisfied.

Hours before the Kremlin official spoke, a U.S. official in Washington said the United States had "talked to our Czech allies and the Russians about a signing in Prague when the treaty is finished."

The U.S. official, who spoke on condition of anonymity, said Prague was "where we always wanted to do a signing."

Reuters