(Adds Russian comment, more details from conference call)
By Sue Pleming
WASHINGTON, Aug 9 (Reuters) - Russia has used "disproportionate" force in the South Ossetia conflict with Georgia and must immediately agree to a cease-fire 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 the breakaway separatist region.
"The response has been far disproportionate from whatever threat Russia was citing," the senior American official told reporters in a conference call. "We are calling for an immediate cease-fire and a stand down of all troops."
The official, who was not authorized to speak on the record by the Bush administration, said Russia was using a giant strategic bomber capable of carrying 54,000 pounds (24,500 kg) of bombs and had launched ballistic missile attacks on Georgian territory.
"I cannot for the life of me imagine how that could be a proportional response," he said. "The only winning option for Russia is to cease the attacks."
President George W. Bush urged Moscow to stop bombing immediately, saying it marked a dangerous escalation.
The senior U.S. official said Russia, which would be isolated by its allies because of its behavior, must accept international mediation, which could include the United States.
"It is simply not acceptable to reject an offer of a cease-fire and international mediation," he added.
Georgia called for a cease-fire on Saturday after Russian bombers widened their offensive to push back Georgian troops seeking control over South Ossetia.
However, the Kremlin said Russia had yet to receive any cease-fire proposal from Georgia. "The Georgian president has not sent any proposals to the Russian president though any channels," a Kremlin spokesman said.
Russian Deputy Foreign Minister Grigory Karasin told reporters Moscow was ready to discuss the situation, including a cease-fire, 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."
Washington plans to send an envoy to join other international mediators in the region to help end the crisis, but the senior U.S. official said it was unclear when the American diplomat would leave.
Georgia is a close ally of the United States and has relied on military aid and training from Washington. Asked whether Washington would offer military assistance in the crisis, the U.S. official said: "We are not talking about anything beyond diplomacy at this point."
The United States has cautioned U.S. citizens over travel to Georgia and the breakaway regions and dependents of U.S. Embassy staff in the Georgian capital Tbilisi were being given help to leave, the official said.
The United States has pushed hard for Georgia to become a member of NATO despite the misgivings of many European allies and strong opposition from Russia.
"Many people speculate one of the reasons Russia has used this disproportionate force against Georgia is precisely because it does not want to see Georgia move towards NATO," the senior U.S. official said.
"That is an understandable perspective by Russia but it is totally unacceptable to us." (Additional reporting by Oleg Shchedrov in Moscow; editing by Chris Wilson)