Russian cosmonaut Mikhail Kornienko, left, shakes hands with US astronaut Scott Kelly after a press conference at Unesco on Thursday. Photograph: Chesnot/Getty Images

A US astronaut and a Russian cosmonaut gearing up for the longest-ever flight on the International Space Station (ISS) said Thursday they were unfazed by any turbulence in their countries political ties.

Nasa spacefarer Scott Kelly and Mikhail Kornienko of the Russian space agency Roscosmos are to launch to the ISS in 2015 for a year-long stay designed to be a test-bed for a future trip to Mars.

The ISS is a rare area of US-Russian cooperation that has not been hit by the Ukrainian crisis, which has prompted Washington to impose sanctions on Moscow.

In space, we rely on each other implicitly for our lives, Kelly, 50, said at a press conference in Paris to showcase the mission.

Any political issues that exist between our countries is something we dont even discuss. Were great friends, were colleagues, were professionals. Thats the way it has to be.

Kornienko, 54, added: There is no borders in space between us.

Its a great example of ... working together, especially for politics on the ground.

Their spell will be the longest stay aboard the ISS since the orbital outpost in space was manned in 2000.

The longest-ever space trip was by cosmonaut Valeriy Polyakov, who in 1994 and 1995 spent 437 days in the Russian-Soviet station Mir.

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US, Russian space travellers: 'There is no borders in space between us'