(Adds Russian reaction)

WASHINGTON, Aug 9 (Reuters) - Russia has used "disproportionate" force in the South Ossetia conflict with Georgia and must immediately agree to a ceasefire with Tbilisi, a senior U.S. official said on Saturday.

Russia and Georgia came into direct conflict over South Ossetia this week after Tbilisi launched an offensive to regain control over its separatist region.

"The response has been far disproportionate from whatever threat Russia was citing," a senior U.S. official told reporters in a conference call on condition of anonymity.

"We are calling for an immediate ceasefire and a stand down of all troops."

The Kremlin said Russia had yet to receive any ceasefire proposal from Georgia. "The Georgian president has not sent any proposals to the Russian president though any channels," a Kremlin spokesman said.

Earlier U.S. President George W. Bush said Russian attacks on Georgia marked a "dangerous escalation" of the crisis. Russian President Dmitry Medvedev told Bush the only solution was for Georgian troops to quit the conflict zone.

Russian Deputy Foreign Minister Grigory Karasin told reporters that Moscow was ready to discuss matters including the ceasefire with the Georgian leadership but named two conditions.

"One is the withdrawal of all Georgian forces from the conflict zone," he said. ""And the second is an immediate signing of a binding agreement on the non-use of force. ... After that we could discuss all further issues." (Reporting by Sue Pleming in Washington and Oleg Shchedrov in Moscow; Writing by Maria Golovnina)

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