TBILISI - U.S. Vice President Dick Cheney vowed on Thursday to stand by Georgia in its showdown with Russia, saying Moscow's military push into the ex-Soviet state was an "illegitimate" act which cast doubt on its reliability.

Cheney, a hawk and one of Moscow's harshest critics, is the highest ranking U.S. official to visit Georgia since Tbilisi tried to retake the breakaway region of South Ossetia by force in early August and was overwhelmed by the Russian military.

"After your nation won its freedom in the Rose Revolution, America came to the aid of this courageous young democracy," Cheney told reporters, referring to the peaceful revolution in 2003 which brought Georgia's pro-western President Mikheil Saakashvili to power.

"We are doing so again as you work to overcome an invasion of your sovereign territory and an illegitimate, unilateral attempt to change your country's borders by force that has been universally condemned by the free world," Cheney said, standing next to Saakashvili in Tbilisi.

"Russia's actions have cast grave doubt on Russia's intentions and on its reliability as an international partner -- not just in Georgia but across this region and, indeed, throughout the international system."

Cheney is on a tour of U.S. allies in the region that started in Azerbaijan and is due to continue on to Ukraine -- like Georgia an ex-Soviet country seeking NATO membership -- later on Thursday, before ending in Italy.

His visit is certain to rile the Kremlin which has accused Washington of fuelling tensions by emboldening Saakashvili, a U.S.-educated lawyer with close ties to the administration of President George W. Bush.

Both Azerbaijan and Georgia are links in the chain of a Western-backed energy corridor bypassing Russia which the West fears could be in jeopardy following the Kremlin's military thrust into Georgia.

WESTERN CONDEMNATION

Moscow has said it acted in Georgia to prevent what it called genocide when Tbilisi launched its military push into pro-Russian South Ossetia on August 7.

The Kremlin subsequently recognised South Ossetia and a second rebel region, Abkhazia, as independent states, drawing condemnation from Washington and Europe.

It has kept troops in a "buffer zone" on Georgian territory, a move the West says violates a French-brokered peace plan. Moscow says its troops provide security and their presence is not at odds with the six-point ceasefire.

Cheney said Washington was "fully committed" to Georgia's bid for membership in NATO, which he called a "defensive alliance" that posed no threat to Russia.

The United States has considered moves to punish Moscow for its intervention in Georgia, such as scrapping a lucrative civil nuclear deal, but it has not announced any sanctions and the West appears to have few options for influencing Russia.

On Wednesday, the United States announced an aid package of more than $1 billion (560 million pounds) to help Georgia rebuild housing, transportation and other infrastructure destroyed in its five-day war with Russia.

Bush has directed federal agencies to expand economic aid for Georgia. To underline U.S. support, Washington is sending the USS Mount Whitney, a sophisticated command warship of the U.S. Sixth Fleet, to Georgia loaded with more than 17 tonnes of humanitarian aid.

The Kremlin has accused Washington of using its navy to play a dangerous game of brinkmanship in the Black Sea