US surveillance Boeing OC-135B aircraft makes emergency landing in eastern Russia. Picture: Alexander Golovko, KP

The military aircraft had taken off after a stopover in Ulan-Ude, in the Republic of Buryatia, when the American crew detected a malfunctioning of the landing gear, said Russian reports. The plane was heading for Yakutsk but opted to make an emergency landing at Khabarovsk airport.

'A foreign aircraft made a forced landing in Khabarovsk. All emergency ground services have arrived on site. The flight landed safely at 3 P.M. local time,' said a statement by an airport official.

Earlier, Russian Defense Ministry Nuclear Risk Reduction Centre officials announced that a US Boeing OC-135B aircraft would conduct an observation flight over Russian territory between July 25 and 30.

There was no official comment from the Eastern Military District on the incident, but an army source cited by Komsomolskaya Pravda Khabarovsk suggested the malfunctioning was 'not coincidental', and perhaps related to recent military exercises in the area.

'They were due to go direct from Ulan-Ude north-northeast to Yakutsk. Just imagine the kind of loop they needed to make to request the landing at approximately the same distance, but to the east?,' said the unnamed source.

The plane seats up to 35 people, including the cockpit and maintenance crew and 'foreign country representatives' and crew members from the U.S. Department of Defense's Defense Threat Reduction Agency (DTRA).

One vertical and two oblique KS-87E framing cameras are used for low-altitude photography approximately 900 metres above the ground, and one KA-91C panoramic camera, which scans from side to side to provide a wide sweep for each picture used for high-altitude photography at approximately 11,000 metres.

The Treaty on Open Skies was signed in March 1992 and became one of the major confidence-building measures in Europe after the Cold War.

It entered into force on January 1, 2002; 34 states are currently party to the treaty, including Russia and most NATO members.

The treaty establishes an unarmed aerial surveillance program whereby signatory states may conduct flights over the entire territory of fellow participants.