* U.S. wants Russia out "now"

* Russia says it will pull back troops within 24 hours

* Row builds over Russian buffer zone in Georgia

By Oleg Shchedrov

SOCHI, Russia, Aug 21 (Reuters) - Russia said on Thursday it would pull back some of its troops in Georgia within 24 hours after Washington demanded they leave "now", but Moscow said it would still keep a force stationed in Georgia's heartland.

In some of Washington's toughest comments to date, the White House declared Russia in violation of its commitments to leave the pro-Western Caucasus mountain state after routing Georgian forces in a war that erupted two weeks ago.

"The withdrawal is not happening very quickly, if it in fact has begun," White House spokesman Gordon Johndroe said. "The withdrawal needs to take place, and needs to take place now."

U.S. impatience has been growing by the day as it waits for a full-scale pullout of troops and weaponry that Russia sent into its small neighbour two weeks ago to counter a Georgian attack on the Moscow-backed South Ossetia region.

A Reuters reporter saw a column of T-72 main battle tanks lumbering across the border from Russia into Georgia -- the first sign of heavy armour being withdrawn from Georgian soil -- but elsewhere Russian forces remained in place.

Russian defence officials said what they called reinforcement troops would be pulled back to within South Ossetia by the end of Friday, and from there withdrawn to Russian soil within 10 days.

But they made a distinction between those troops and what they described as a peacekeeping force. This force will stay on indefinitely in South Ossetia, and a buffer zone around it, the officials said.

That would leave Russian troops still inside the Georgian heartland and close to the main east-west highway on which its economy depends.

Georgian President Mikheil Saakashvili, who is backed by the United States and wants to take his ex-Soviet state into the NATO alliance, said he would not stand for that.

"There will be no buffer zones. We will never live with any buffer zones. We'll never allow anything like this," he told Reuters in the Georgian capital, Tbilisi.

PULLBACK

Russia says it needs to maintain a force in Georgia to prevent further bloodshed and protect South Ossetians -- most of whom hold Russian passports -- from Georgian attacks. Tbilisi says Moscow is trying to annexe its territory.

Russian defence officials said they were honouring their commitment to pull back under a ceasefire deal brokered by French President Nicolas Sarkozy.

"All Russian military units which were supporting peacekeepers in the zone of the Georgian-Ossetian conflict will start pulling out from Georgian territory to South Ossetia on Aug 22," the defence ministry said.

"In the course of August 22 the withdrawal of these units from Georgian territory will be completed."

"Within the security zone established in accordance with the principles of settlement, which Russia is meticulously following, only peacekeepers at special checkpoints in the quantities needed to ensure security will remain," it said.

The crisis erupted on Aug. 7-8 when Georgia tried to retake South Ossetia, a pro-Moscow region which is ethnically distinct from Georgia and broke with Tbilisi in the early 1990s.

Russian forces hit back, thrusting beyond the region deep into Georgia and overrunning the army in fierce fighting.

NATO states have pressed Russia to pull its troops swiftly out of Georgia and the alliance this week froze contacts with Russia over the conflict.

Russia responded by saying it was suspending military cooperation with Latvia, Estonia and Norway.

AFGHANISTAN

It was unclear if there would be any impact on a crucial aspect of NATO-Russian cooperation: the deal under which Moscow allows aircraft supplying the NATO-led force in Afghanistan to fly through Russian airspace.

Russian Foreign Minister Sergei Lavrov said Moscow was "not going to shut the doors" on cooperation with NATO, but he pointedly raised the issue of Afghanistan transit.

"After the famous NATO meeting (when the alliance froze contacts with Russia), some leading alliance officials were whispering in my ears: 'You are not going to halt the Afghanistan transit, are you?'", he said.

Underlining Western support for Georgia, a top U.S. general said the Pentagon expected to help Tbilisi rebuild its military, which was left crippled by the Russian attack.

A U.S. warship will on Friday head into the Black Sea -- an area where Russia is sensitive about the presence of NATO forces -- to deliver relief supplies to Georgia, the U.S. navy said.

Valery Gergiev, Russia's best-known living conductor and an ethnic Ossetian, held a concert in South Ossetia on Thursday designed to focus the world's attention on what he said was the devastation Georgian forces had inflicted on the region.

But Georgia has accused Russia and their South Ossetian separatist allies of exploiting the war to drive ethnic Georgians out of their homes and torch their villages. (Additional reporting by David Brunnstrom in Tbilisi, Dmitry Solovyov in Tskhinvali and Tabassum Zakaria in Crawford; writing by Christian Lowe; editing by Andrew Roche)

For special cover see: http://www.reuters.com/news/globalcoverage/georgiaconflict For Reuters blogs see: http://blogs.reuters.com/global/

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