Two US F-22 Raptor fighters have been flown to Mihail Kogalniceanu air base in Romania to deter Russian agression REUTERS/Toby Melville

The US Air Force has sent two F-22 Raptor fighter jets to Romania to deter further Russian intervention in Ukraine in the latest escalation of tensions on Nato's eastern border. It comes as Europe's Eastern European members requested from Nato a stronger presence in the region following Russia's annexation of Crimea.

That seizure and subsequent conflict in Eastern Ukraine have led to sanctions by the US and Eastern members of the Nato bloc including former Soviet-ruled Baltic states which have also lobbied the organisation to step up its presence in the Baltic.

Earlier in April, Poland's foreign minister even said that Vladimir Putin's Russia is an "existential threat" that is more dangerous than Isis after two Russian SU-24 jets made "simulated attacks" on the American warship the USS Donald Cook.

The cutting-edge fighter jets landed at Mihail Kogalniceanu air base, close to the Black Sea port of Constanta in south-east Romania, on Monday 25 April. The aircraft which are virtually unable to be detected by radar, possess sophisticated sensors allowing pilots to track, identify, shoot and kill air-to-air threats.

They can also attack surface targets and they are so effective the US Congress banned constructors Lockheed Martin from selling them abroad. Twelve F-22s are stationed at RAF Lakenheath, in Suffolk as part of the Operation Atlantic Resolve, which contributes to Nato's collective security and regional stability in Europe.

Romania has been a Nato member since 2004 and Air Force head Major General Laurian Anastasof said planes would be scrambled should unidentified aircraft come within 20 miles (32km) of Romanian airspace, which is something he says has happened four times this year.

US Ambassador to Bucharest, Hans Klemm, said the US and Romania were seeking to improve ties, telling Sky News that there needed to be better security in south eastern Europe "as a result of the aggression by Russia that has brought so much instability to this part of the world over the past two to three years".