WASHINGTON (CNN) -- Russia's use of strategic bombers and ballistic missiles against Georgia's civilians outside of the South Ossetian conflict is "far disproportionate" to Georgia's alleged attack on Russian peacekeepers, a senior U.S. official said Saturday.

The official was not authorized to speak on the record due to the sensitive nature of the diplomacy.

Russia's use of its potent air weaponry signals a "severe" and "dangerous escalation in the crisis," the official said.

"For the life of me, I can't image that being a proportionate response to the charge that Georgia has attacked Russian peacekeepers," the official said. "It's hard for us to understand what Russia's plan is here."

The official said Russia is probably trying to destabilize Georgia politically to kill its chances of joining NATO.

Georgia wants to join NATO, but Russia opposes the move, concerned that the alliance's eastward march will erode its influence. NATO rejected Georgia's membership bid in April despite strong lobbying from U.S. President Bush, though the alliance promised that Georgia could join at a later date. Watch President Bush comment on conflict »

Don't Miss Armor streams into South Ossetia as refugees flee

Armor streams into South Ossetia as refugees flee Russia: Troops in South Ossetia to enforce peace

Russia: Troops in South Ossetia to enforce peace Russian warplanes target Georgia

Still, Georgia bears some of the blame for the fighting over South Ossetia, the official said. Recently, the United States has had "very blunt exchanges with" Georgia, telling its leaders that they have no chance of winning a war with Russia and that they should stick to a path of diplomacy. Watch injured Georgian soldier being treated »

The official said European allies have told the United States that Russia has "crossed a line of unacceptable behavior" and should "expect international condemnation."

"I do sense an emerging unified view among our key allies," he said.

The United Sates, Britain and NATO on Friday called for a cease-fire. And on Saturday, French President Nicolas Sarkozy proposed an immediate cease-fire as part of a three-step plan to end fighting.

Sarkozy's proposal calls for the return of Russian and Georgian troops to their former positions and requires Georgia's sovereignty and territorial integrity be respected. A statement from Sarkozy's office did not provide further details on the plan.

The official also said the State Department has authorized only the voluntary departure of dependents of U.S. embassy employees from the Georgian capital of Tbilisi. The official said he did not know how many dependents would choose to leave.

Meanwhile, the U.N. Security Council held a private meeting on Georgia on Saturday, though it seemed unlikely that there would be any immediate agreement on a statement on the conflict.

The U.S., European Union and international security organizations on Friday called for an end to the fighting

Bush and Russian Prime Minister Vladimir Putin on Friday discussed the conflict in Beijing, where they attended the opening of the Summer Olympics.

The EU and the Organization for Security and Co-operation in Europe were sending envoys to Georgia to secure a cease-fire, but a senior U.S. State Department official said the United States would send only a representative after a cease-fire is in place.

The European Union said it was working with other parties "towards a ceasefire in order to prevent further escalation of this conflict." EU spokeswoman Christina Gallach told CNN: "We think it is not acceptable to see these scenes of bloodshed and destruction."

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