US 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.

Meanwhile, Georgian police have arrested a son of Georgia's first post-Soviet leader Zviad Gamsakhurdia on charges of trying to overthrow pro-Western President Mikheil Saakashvili. A senior government source said Tsotne Gamsakhurdia was arrested at Tbilisi airport.

Cheney, a hawk and one of Moscow's harshest critics, is the highest ranking US 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,” he said, standing next to Saakashvili in Tbilisi.

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

Cheney is on a tour of US 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 US-educated lawyer with close ties to the administration of President George W. Bush.