In a brief statement, the National Aeronautics and Space Administration said flight controllers informed the station crew that they were monitoring "a slightly larger than normal decay in air pressure on the complex."

"The decay, which is currently measured at a rate of less that 0.04 pounds per square inch a day, is having no impact on station operations and the crew is in no danger," NASA assured.

The agency did not say when a drop in air pressure was first noticed, but NBC News cited a confidential internal NASA report as saying it came to the attention of mission control on December 29.

US mission commander Michael Foale and Russian flight engineer Alexander Kaleri spent some time Monday inspecting valves in both the US and Russian components of the station, but found "nothing unusual" and went to sleep, according to space officials.

Kaleri also conducted an inspection of windows in the Russian-made Zvezda service module and a docking compartment, NASA said. He was expected to later report his findings to Russian space experts.

The Russian engineer has experience in dealing with orbital emergencies gained in 1997 when he was working on the now defunct space station Mir, which was rammed by a Russian resupply ship during a botched docking attempt.

The two men -- along with flight controllers in Houston, Texas -- were expected to continue what officials called "troubleshooting activities" on Tuesday.

Forty-six-year-old Foale and Kaleri, 47, arrived on the ISS aboard a Russian Soyuz spacecraft last October.

The two astronauts are planning to spend aboard the station a total of nearly 200 days, studying microgravity environment in low-Earth orbit and preparing for this year's arrival of the cargo ship Jules Verne, built by the European Space Agency, that will be making its maiden voyage to the station.

The crew is scheduled to receive a Russian resupply mission later this month.

The announcement of diminishing air pressure follows a still largely unexplained incident in late November, when the astronauts reported a loud noise similar to that produced by a can being crushed, according to Russian space officials.

The report prompted speculation the station might have been hit by a piece of space junk. But after careful examination, the astronauts discovered no evidence of that and reported no change in air pressure.

NASA said the drop in air pressure had not stopped research activities aboard the spacecraft.

On Monday, Foale and Kaleri participated in a Russian medical experiment designed to study changes in the human body during space flight, according to the agency. Foale also did some preparatory work for future spacewalks.

The ISS was launched in 1998 as a collaboration between 16 partners including the United States, Russia, the European Union, Japan and Canada.

SPACE.WIRE