Russian space officials have formed a group of specialists to investigate a slow but steady drop in air pressure aboard the International Space Station. The leak was first noticed on New Year's Day and officials insist the problem is not a threat to the two men now on board.

The Russian space agency has set up a working group of experts at mission control outside Moscow to look into the possible cause of the pressure leak.

Agency officials say the leak is small and the drop in pressure has been very gradual. But they say the source of the problem must be found and fixed.

Konstantin Kreidenko, a spokesman for the agency, says oxygen supplies on the ISS are sufficient and that the two-man crew is not in any danger. Russian cosmonaut Alexander Kaleri and U.S. astronaut Michael Foale are the two men now on board.

Mission control in the United States first detected the problem last week and informed the crew of it on Monday, saying there was nothing they could do right away about the problem. Later, Mr. Foale radioed back to say he and his colleague had checked the valves that could easily be reached and found nothing wrong.

Russian officials say the plan now is to have the men close off each of the station's various compartments one-by-one to check for air tightness. This would narrow down the search for the leak on the giant station, which consists of many different modules that have been interlinked over several years.

Russian engineers built some of the modules and have identical mock-ups on earth that can be checked.

Some specialists say the cause of the leak may be a faulty valve where the cargo supply ship is docked to the station, and that fixing it may involve checking hundreds of valves and sensors.

Russian space experts dealt with these kinds of problems on the Mir station that had orbited earth for more than a decade before it was replaced by the ISS.

The two current ISS occupants boarded the station last October and are scheduled to remain until April.