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Russia sent troops and dozens of tanks and armoured vehicles into the breakaway Georgian province of South Ossetia today, vowing to protect its citizens in a move described by Tbilisi's pro-Western Government as an act of war.

A South Ossetian rebel minister said that more than 1,000 people had been killed in overnight shelling of the city of Tskhinvali, the separatist capital which Georgia claimed today to have captured.

In probably the most serious regional crisis since the collapse of the Soviet Union in 1991, at least 50 Russian tanks – and possibly many more – rumbled through the Roki tunnel, which cuts through the Caucasus mountains separating South Ossetia from the Russian province of North Ossetia.

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The Russian incursion came after Georgian forces moved to regain control of the region that broke away along with rebel Abkhazia in the 1990s. Russia backs the separatists in both provinces and has its own peacekeeping forces there.

"One hundred and fifty Russian tanks, armoured personnel carriers and other vehicles have entered South Ossetia," President Saakashvili of Georgia told reporters in Tbilisi. "This is a clear intrusion on another country’s territory. We have Russian tanks on our territory, jets on our territory in broad daylight."

Mr Saakashvili added that Georgian forces had downed two Russian jet fighters over Georgian territory.

Georgia mobilised its reservists yesterday and launched a massive offensive overnight. Fighting raged today around the city of Tskhinvali, the regional capital, as Georgian troops backed by tanks and warplanes pounded separatist forces,

Eduard Kokoity, the South Ossetian rebel leader, said that there were "hundreds of dead civilians" in the city. His Nationalities Minister, Teimuraz Kasaev, put the number at "more than a thousand".

A senior Russian military commander said parts of Russia’s 58th army were approaching the rebel capital.

"We cannot allow the deaths of our countrymen to go unpunished. The guilty parties will receive the punishment they deserve," President Medvedev told a meeting of his security council in the Kremlin.

"I am obliged to protect the lives and dignity of Russian citizens, no matter where they are located."

Mr Medvedev's predecessor, Vladimir Putin, now Prime Minister, warned on a visit to Beijing, where he attended the Olympic opening ceremony, that the Georgian action would bring "retaliatory measures".

Both Nato and the White House appealed for an end to the fighting as international diplomacy went into overdrive and Mr Putin discussed the crisis with President Bush at a gathering ahead of the Olympic ceremony. A US official said that 100 American military trainers working in Georgia were all safe.

The International Committee of the Red Cross called for the establishment of a "humanitarian corridor" through South Ossetia.

South Ossetian officials said that much of Tskhinvali had been destroyed in the Georgian offensive. A Reuters correspondent some two miles from the city said that the roar of warplanes and the explosions of heavy shells was deafening and many houses were ablaze.

The area is of strategic importance, largely because of the BTC oil pipeline, which runs through central Georgia just south of the breakaway region. The pipeline – which features in the 1999 James Bond film The World is Not Enough – pumps around one per cent of global crude supplies from the Caspian to the Turkish port of Ceyhan for export to Western Europe but is already closed because of an attack in Turkey last week by the Kurdish separatist organisation PKK.

Mr Saakashvili, a US-educated lawyer who succeeded Eduard Shevardnadze in 2004 and has since tried to align it more closely to the West, compared the Russian action with the invasion of Afghanistan in 1979 and appealed to the outside world to intervene.

"Russia is fighting a war with us in our own territory," he told CNN as Russian armour rolled into South Ossetia.

"It’s not about Georgia anymore. It’s about America, its values: we are a freedom-loving nation that is right now under attack."

Tensions between Georgia and Russia have been rising over the last few months over South Ossetia and Abkhazia.

Georgia accuses Russia of fermenting trouble in both regions and supporting the separatist governments as a way to put pressure on Georgia and foil its attempts to join Nato. Russia has given out passports to a majority of South Ossetians and Abkhazians.

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