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The six Sukhoi jets are armed with live missiles, sparking fears we could be on the verge of a new Cold War face-off.

Nato has ordered the huge Awacs radar plane to monitor Russia’s Black Sea fleet and army activity on the ground.

The plane, based on a Boeing 707 airliner, can fly direct from RAF Waddington in Lincolnshire to loiter high in the sky listening to “electronic noise” in the area.

Crews are under strict orders to remain within Polish and Romanian airspace to avoid any conflict with the Russians.

But Aleksandr Lukashenko, the president of Belarus and a close ally of Moscow, requested help against what he claimed was a potential Nato threat and the Sukhoi SU-27s were sent straight to his country.

Codenamed Flanker by Nato, the SU-27s, among the most advanced fighter aircraft in the world, flew in carrying live missiles.

The Russian aircraft are expected to patrol near the Polish and Romanian borders within their airspace but there are fears they could harass Nato aircraft.

The top secret RAF mission involves 40 UK personnel.

A senior RAF officer said: “We are there on behalf of Nato to monitor what the Soviets are doing and what, if any, military activity there is this weekend as the referendum takes place.”

Nato gave the go-ahead for the spy flights last Monday.

A Nato spokesman said last night: “Awacs reconnaissance flights will take place solely over alliance territory.

“As the UK E-3D Awacs are part of the Nato Awacs force, then it is only right that the UK undertakes its share of this mission.”

Yesterday President Putin’s Russia vetoed a United Nations call which criticised today’s vote on the future of Crimea.

Locals are expected to vote for it to become part of Russia again – a move which Ukraine and the West says is illegal.