By Denis Dyomkin and Susan Cornwell

MOSCOW/WASHINGTON (Reuters) - Russia and the United States have reached agreement on a landmark nuclear arms reduction treaty, the Kremlin said on Wednesday, but the White House said 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 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 again."

"There are still some things that need to be worked out," Gibbs told a daily news briefing, adding he expected the two leaders to speak to each other in the next few days.

Asked later about Gibbs' statement, a Kremlin official who spoke on condition of anonymity said, "The deal is agreed on the whole, but there are some technical details that still need to be resolved."

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

The White House hopes to sign the new treaty on April 8, said Republican Senator Richard Lugar after he and Democratic Senator John Kerry were briefed by Obama.

"April 8 in Prague, that's at least the hope that I think they have," said Lugar, the senior Republican on the Senate Foreign Relations Committee that will handle the treaty.

That date would be around the anniversary of Obama's April 5, 2009, speech in Prague offering his vision for reducing global nuclear arsenals.

"The president is fairly confident that he will be heading toward a signing," Lugar said. He told Reuters he believed he would be able to support ratification, which requires a two-thirds Senate majority.

EFFORT TO IMPROVE RUSSIA-U.S. RELATIONS

For almost a year, Russian and U.S. negotiators have tried to reach a successor to the 1991 Strategic Arms Reduction Treaty. They missed a December 5 deadline when START I expired.

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

It could also strengthen Obama politically, giving him a major foreign policy success and building on the domestic political victory he scored this week when he signed sweeping healthcare reform into law.

"I don't think the discrepancy (in the statements from Moscow and Washington) is significant in the long term," said Greg Thielmann, a former State Department analyst and senior fellow at the Arms Control Association in Washington.

"We are on the verge of getting another very significant reduction in nuclear forces that is verifiable and helps put U.S.-Russian arms control back on track," he said.

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, with a more specific ceiling to be determined in the negotiations.

The most recent treaty to cut the Cold War foes' nuclear weapons numbers, signed by former Presidents George W. Bush and Vladimir Putin in 2002 before ties deteriorated, limited each side to 2,200 warheads each by the end of 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 that have or want atomic weapons and 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 rid the world of nuclear weapons. He is due to host a summit on nuclear nonproliferation in Washington next month.

Analysts say the treaty is also in the Kremlin's interests, estimating that Russia's 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 disputes on issues ranging from other weapons to trade and human rights.

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

Lugar said missile defense would not be part of the treaty. "However, it may be dealt with in other ways, that is mention of it, and the relationship between offense and defense as both countries view their situations."

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 favor of Washington.

Any limit on U.S. missile defenses in the START successor would jeopardize its chances of approval by the U.S. Senate.

Kerry, Foreign Relations Committee chairman, said he would push for Senate action this year once the pact is signed.

Last week, the Senate's two top Republicans -- Mitch McConnell and Jon Kyl -- wrote a letter reminding Obama of a requirement in recent legislation that the administration submit a 10-year plan for modernizing the U.S. nuclear arsenal at the same time it submits the new START deal to the Senate.

Russia's lower house of parliament, dominated by Putin's United Russia party, must also ratify the treaty for it to enter force.

(Additional reporting by Ross Colvin and Patricia Zengerle; Writing by Steve Gutterman; Editing by Tim Pearce and Peter Cooney)