The conflict in the Caucasus today spread to Georgia's second breakaway province of Abkhazia, where separatist rebels and the Russian air force launched an all-out attack on Georgian forces.

Abkhazia's pro-Moscow separatist leader Sergei Bagapsh said his troops had launched a major "military operation" to force Georgian troops out of the mountainous Kodori gorge, which Georgian forces control as a strategic foothold in the breakaway Black Sea territory.

He said "around 1,000 special Abkhaz troops" were involved. They were attacking and pounding Georgian positions using "warplanes, multiple rocket launchers and artillery", he said.

"The operation will enter the next phase as planned. And you will learn about that," he promised today, adding that he would create a "humanitarian corridor" allowing residents living in the district to flee.

The offensive appeared to mark a dangerous new front in the conflict between Georgia and Russia - following Georgia's apparent withdrawal from its other breakaway region of South Ossetia today.

Georgia immediately accused Russia of planning and executing the attack on the Kodori valley - a small but strategic enclave just inside Abkhazia, and controlled by Georgian forces since 2006.

Georgian interior ministry spokesman Shota Utiashvili said: "They have started the operation to storm Kodori gorge." Asked who was behind the operation, he replied: "The Russian army."

Speaking in Tbilisi, Georgia's parliamentary speaker, David Bakradze, said he had "irrefutable proof" that the Russian military was masterminding the "plan". He urged residents to stay calm, adding "the enemy will be offered all resistance".

In an interview on BBC World, the Georgian president, Mikhail Saakashvili, accused Russia of having 150 tanks and 10,000 troops in Abkhazia.

Georgia earlier said Russia had landed 4,000 troops in Abkhazia last night - a subtropical exclave on the eastern coast of the Black Sea, which abuts the Russian resort town of Sochi, where Vladimir Putin has his summer residence.

It added that 10 Russian jets had pounded Tbilisi-controlled 'upper Abkhazia' on Saturday, and that Georgian government troops had shot one of them down.

The Russian air force bombarded Chkhalata - the gorge's main administrative centre, where Georgia has installed an alternative pro-Tbilisi Abkhaz government. It also hit the airdrome village of Omarishara, Georgia said.

Russia today insisted it did not want to "escalate the conflict". Army spokesman Anatoly Nogovitsyn told the agency Interfax: "We do not intend to take the initiative in escalating the conflict in this region. We are primarily interested in stabilisation [in Abkhazia]."

But there seems little doubt that Russia is determined to evict Georgian troops from the Kodori gorge. Its aim appears to be to restore the de facto border between Georgia and Abkhazia - and to boost the pro-Russian separatist government based in the seaside town of Sukhumi.

Abkhazia has enjoyed de facto independence since driving Georgian forces out in a 1992-3 civil war. Like South Ossetia, it has received help and assistance from Russia, with most residents now holding Russian passports.

Vladimir Putin recognised Abkhazia as a legal entity in April - a move that enraged Tbilisi, which accused him of trying to annex the territory.

The UN secretary general Ban Ki-Moon said he was profoundly concerned over growing tensions in Abkhazia.

Ukraine also warned that it might not allow Russian ships deployed off Abkhazia to return to their base in the Crimea.