The White House said Russia's military action in the Georgia conflict was "dangerous and disproportionate" and warned it could have a "significant long-term impact" on relations between Washington and Moscow.
"We're alarmed by this situation," James Jeffrey, US President George W. Bush's deputy national security adviser, told reporters in Beijing where the president was attending the Olympics, referring to reports of a Russian naval blockade.
He said the United States had made it clear to the Russians that their continued escalation in the conflict with Georgia over its breakaway South Ossetia province could seriously harm U.S.-Russia ties.
Mr Jeffrey also said the United States would be "very, very concerned" if reports of Russian ground attacks in Georgia, a US ally, were accurate.
The Ukraine said it reserved the right to temporarily bar Russian warships dispatched to the Georgian coast from returning to their Ukrainian base of Sevastopol.
"Ukraine ... reserves the right to bar warships and vessels which could take part in the action (conflict with Georgia) from returning to Ukrainian territory until the conflict is solved," a Ukrainian Foreign Ministry statement.
"The Ukrainian side warns the Russian side against the possibility of the Black Sea warships taking part in the conflict over South Ossetia," said the statement posted on the ministry website.
Russia has accused ex-Soviet Ukraine, a strong pro-Western ally of Tbilisi, of supplying weapons to the Caucasus state and encouraging it to strike South Ossetia.