A United States 'spy plane' has made an emergency landing in eastern Russia, it has emerged today.

The surveillance Boeing OC-135B aircraft was flying a mission over Siberia as allowed under the Treaty on Open Skies when it reported a problem with its landing gear.

The unarmed plane made an emergency landing at Khabarovsk airport, but a military source in Russia has questioned whether the technical glitch was genuine.

Under the treaty, signatories are allowed to overfly the skies of each other gathering information about military forces and activities of concern to them.

The American military aircraft had left after a stopover in Ulan-Ude, in the Republic of Buryatia, and was due to fly north to Yakutsk, capital of Siberia's diamond-rich Sakha Republic.

Instead, after take-off the crew noticed the problem and the Boeing went east and made a landing 1,660 miles away in Khabarovsk.