US mission commander Michael Foale discovered the leak in the US module early on Monday and repaired it with Russian flight engineer Alexander Kaleri, the officials told news agencies.

Pressure aboard the station had been slowly dropping for several days and on Monday morning stood at 13.92 pounds per square inch (psi), Russian space officials told news agencies.

After the crack was sealed, pressure climbed back up to 14.11 psi, which is considered normal, they said.

"The crew will isolate the American Destiny module for a period of time on Wednesday from other sections of the craft to make sure that it is hermetic," Pavel Vinogradov, the cosmonaut chief at the Energia corporation, told the ITAR-TASS news agency.

On Monday, the astronauts began to transfer food, water and documentation from American Destiny to the Russian section, Vinogradov said.

The US module will be sealed for at least 24 hours as the astronauts measure air pressure inside to make sure there are no other leaks, he said.

"We have to make sure that this was the only place of air loss," Vinogradov said.

Foale was to move into the Russian part of the station while the US one remained sealed off, he said.

A NASA representative in Moscow downplayed the seriousness of the move, saying that technical experts were "80-percent certain" they had now identified and repaired the leak, which had been in a flex hose inside the US part of the ISS.

"In any vessel, if you are concerned about the pressure integrity you have to isolate the compartment," NASA's Jim Neuman told AFP.

"The flex hose has been removed and the leak has gone," he added.

Monday's development followed a drop in cabin pressure aboard the ISS last week.

NASA (the US National Aeronautics and Space Administration) had earlier assured that the loss of pressure, measured at a rate of less that 0.04 pounds per square inch a day, was "having no impact on station operations and the crew is in no danger".

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

Foale, 46, and Kaleri, 47, arrived on the ISS aboard a Russian Soyuz spacecraft last October.

The two astronauts are planning to spend nearly 200 days aboard the station, studying the 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, on its maiden voyage to the station.

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

The recent falls in air pressure came after 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 that 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.

SPACE.WIRE