by CHRISTIAN GYSIN

Last updated at 23:16 20 July 2007

RAF jets were scrambled twice yesterday to intercept Russian aircraft heading for and inside British airspace.

Tornado fighters had already been sent up on Tuesday to meet two bombers approaching Britain from a Russian base in the Arctic Circle.

A Ministry of Defence spokesman said it was "rare and unusual" for three incidents to follow so closely.

The aerial tensions come amid titfortat expulsions of diplomats following the Kremlin's refusal to extradite radiation murder suspect Andrei Lugovoy.

Russia's foreign minister, Sergei Lavrov, said last night he wanted to see a return to "normal relations" with Britain.

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But the arrival in yesterday's early hours of the Tupolev aircraft - also known as Bears - suggests president Vladimir Putin is not backing down.

Two Tornados were scrambled from RAF Leeming, near Darlington, after two Tu95 bombers were picked up on radar. They were intercepted inside British airspace shortly after 2am but, with the RAF pilots on their tail, the intruders turned and flew north.

Another alert a few hours later led to the scrambling of two more Tornados. RAF sources said the planes were recalled when it turned out that the Tu160 Blackjack bomber was only on the "fringes of UK airspace". It also made off to the north from a position off Scotland.

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Tuesday's incident saw two Tu95s approached by Norwegian F16s and RAF Tornados off the coast of Norway.

Russian air force commander Colonel Alexander Zein said the encounter had no connection with the political row between London and Moscow.

"Our planes were flying planned flights over neutral waters," he insisted. Yesterday a Russian air force spokesman said all the sorties were training flights.

The Tu95 can carry missiles as well as undertake surveillance missions.

With a range of more than 8,000 miles at a top speed of 575mph, it was the Russian answer to the American B-52. The Tu95 took its maiden flight in the early 1950s and was designed to drop nuclear weapons.

It has been redesigned to carry cruise missiles and can even be converted for use as a civilian airliner.

The Tu160, which can reach speeds of up to 1,380mph, was brought into service in the 1980s.

Nicknamed the White Swan by its Russian pilots, it can carry up to 88,000 lbs of ordnance including either cruise or short-range nuclear missiles.

Russian authorities would not disclose whether the planes in this week's incidents were carrying bombs or simply on surveillance duties.

On a visit to Paris yesterday, Gordon Brown insisted that Mr Lugovoy, the prime suspect in the murder of exiled dissident Alexander Litvinenko, must be extradited to face trial in the UK.

Russia has insisted however that it can deal with the case through its own justice system.

Mr Litvinenko, who like Mr Lugovoy was a former agent of the FSB - the successor to the Communist-era KGB - died in a London hospital last November from a fatal dose of the extremely rare radioactive isotope polonium-210.

A British citizen, he was a fierce critic of Mr Putin's regime and had close links with a network of expatriatedissidents, both in the UK and New York.

He and Mr Lugovoy met at the Millennium Hotel in London's Grosvenor Square on the day he was poisoned.

Mr Brown said: "We had to take difficult decisions in relation to the Litvinenko case and we will not tolerate a situation where a British citizen is assassinated on British

"Our first duty is to protect our citizens and to prevent there being lawlessness in the streets of London."

Britain announced on Monday it was expelling four Russian diplomats from London.

Moscow retaliated on Thursday, saying four Britons would be kicked out of Russia.

The RAF tried to play down suggestions that the three separate incidents involving Russian planes were linked to political tensions.

Mr Lugovoy yesterday claimed that the British secret services attempted to recruit him in a spying mission to gather compromising material about the Mr Putin.