(Releads with cancellation)

By Andrew Gray

WASHINGTON, Aug 12 (Reuters) - The United States has canceled a naval exercise with Russia in protest over Moscow's military operations in Georgia, a senior U.S. defense official said on Tuesday.

Top U.S. officials, including President George W. Bush, have strongly criticized Russia's military campaign in Georgia but the decision to cancel the exercise was among Washington's first concrete actions of protest.

"There is no way in good conscience that we could proceed with a joint naval exercise given the state of this crisis," the defense official told Reuters, speaking on condition of anonymity as no official announcement has yet been made.

The annual exercise is led by the United States and known as FRUKUS after the initials of its participant countries -- France, Russia, Britain and the United States, Pentagon officials said.

It was due to begin within the next week in the Pacific Ocean off the coast of the Russian port city of Vladivostok, the officials said. Two U.S. ships were scheduled to be involved, one official said.

Fighting between Russian and Georgian forces began last week after Georgia tried to regain control of the separatist province of South Ossetia and Russia launched a massive counter-offensive.

Russian President Dmitry Medvedev ordered a halt to military operations in Georgia on Tuesday but Tbilisi cast doubt on the announcement, saying Moscow was still bombing towns and villages.

The United States has made clear that it is not contemplating any use of force to support Georgia, a strong U.S. ally.

But officials say military planners are looking at a range of options to provide humanitarian aid if requested to do so.

The United States has less than 100 military personnel in Georgia, most of them involved in training Georgian forces. The Pentagon has said they are playing no role in the current conflict. (Additional reporting by Susan Cornwell) (Reporting by Andrew Gray, editing by Patricia Zengerle)

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