Russia began pulling tanks and troops back over its frontier, pledging all forces would quit Georgia's heartland by tonight, but the United States demanded it leaves "now".
In some of Washington's toughest comments to date, the White House declared Russia in violation of its commitments to leave the territory of Georgia after routing Georgian forces in a war that erupted two weeks ago.
White House spokesman Gordon Johndroe said he could not imagine a circumstance in which the US would engage in military-to-military cooperation with Moscow until the Georgia situation was resolved.
US 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 Caucasus neighbour two weeks ago to counter a Georgian attack on the Moscow-backed South Ossetia region.
A new dispute was building over Russian plans to establish a buffer zone around South Ossetia. That would leave Russian troops still inside the Georgian heartland and close to the main east-west highway on which its economy depends.
A Reuters reporter at the Roki tunnel, a few kilometres from the Georgian-Russian border and the main access route for Russian forces, said he had seen the first armoured vehicles return to Russia since the pullback was due to start days ago.
"I can see 21 T-72 tanks moving towards the Roki tunnel in the direction of Russia," he said.
"I can also see four Grad artillery launchers, several armoured personnel carriers, and heavy trucks ready to move into the tunnel," the reporter said.
In Moscow, Anatoly Nogovitstyn, deputy chief of the military's General Staff, told a news briefing that Russian troops would complete their pullback by today.
Georgia's Western allies were not convinced. "There has not been much evidence of any significant Russian withdrawals," Pentagon spokesman Bryan Whitman said in Washington. "There have been some minimal movements to date."
Russian Foreign Minister Sergei Lavrov said the Kremlin planned to keep 500 troops in a buffer zone, or "zone of responsibility" which includes territory outside South Ossetia.
Officials in Moscow said that was in keeping with a French-brokered ceasefire deal, and agreements on South Ossetia dating back to the 1990s.
"These principles allow the Russian peacekeepers to take additional measures to ensure security in the security zone," Mr Lavrov told reporters.
But Georgian President Mikheil Saakashvili, who is backed by the US and wants to take his tiny 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.
Speaking earlier at a news conference, Mr Saakashvili said that far from pulling back, the Russian army if anything was widening the areas it occupied.
"They don't show any sign that they want to give up control," he said. "It looks like the word 'withdrawal' is understood in different ways by different people."
The crisis erupted on August 7-8 when Georgia tried to retake South Ossetia, a pro-Moscow region which broke with Tbilisi in 1992. 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.
Mr Lavrov said Russia had no plans to "shut the doors" to Nato in retaliation, but he reminded partners in the alliance they had much to lose if they ended cooperation with Russia.
Many of the supplies to the Nato security force in Afghanistan are flown in through Russian airspace under a deal with Moscow.
"I can only say that Russia needs cooperation with Nato no more than Nato needs Russia," he told reporters.
Underlining Western support for Georgia, a top US general said the Pentagon expects to help Tbilisi rebuild its military, which was left crippled by the Russian attack.
Valery Gergiev, Russia's best-known living conductor and an ethnic Ossetian, was holding a concert in South Ossetia last night 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 its South Ossetian separatist allies of exploiting the Georgian defeat to drive ethnic Georgians out of their homes in the region, and torching their villages.
During a visit to the Georgian city of Gori, near the breakaway region, Alexander Stubb, Finnish Foreign Minister and chairman of the Organisation for Security and Cooperation in Europe, accused Russian forces of mistreating local people.
"What we have just seen is atrocious. There are Russian Interior Ministry special forces bringing old people from villages, basically emptying the villages on the South Ossetian side and dumping them in Gori," he told Reuters.