Alexander Medvedchikov and Alexei Krasnov, two high-ranking officials within the Russia Space Agency, had been due to leave on Friday for a conference in Monterey, California.

"Even though all of the necessary documents were handed to the US embassy on time, Medvedchikov and Krasnov were refused the US visas," agency spokesman Sergei Gorbunov told AFP.

"The embassy did not explain their decision," Gorbunov said. "Russia's delegation will have to go to the United States short-staffed, and that may have a negative impact on the talks."

He added: "It is not the first time that such a thing has happened."

US embassy officials were not immediately available for comment.

Russia is a key partner along with the United States, European Union, Canada and Japan in the ISS project.

Following the February 1 US shuttle disaster, Russian craft provide the only link to the station.

SPACE.WIRE