NASA has reopened sections of the International Space Station (ISS) sealed off after what has now been confirmed to be a false alarm over an ammonia leak.

The US agency confirmed midday Wednesday, Pacific Time, that the crew had been cleared to reenter the American-built wing of the orbiting lab, and that no evidence of any contamination was found. A computer part had cocked up, and power cycling the thing ended the drama, we're told.

The astronauts had been ordered to leave and seal off the US-run part of the research station due to an apparent leak of dangerous gas from coolant pipes. The crew moved to the Russian section of the station while the lab's software shut down the cooling system.

While NASA had said early on that it believed the incident to be a false alarm, it warned that the closed portion could be sealed off for some time however, the 'nauts are now free to move and work around the whole station.

"The alarms this morning that initiated the movement of the crew out of the U.S. segment are suspected to have been caused by a transient error message in one of the station’s computer relay systems, called a multiplexer-demultiplexer," the agency said.

"A subsequent action to turn that relay box off and back on cleared the error message and the relay box is reported by flight controllers to be in good operating condition."

While there is no longer thought to be any risk of danger, NASA said the crew will continue to wear protective masks throughout the day as it brings the cooling system back online and runs tests.

NASA said that it expects to resume normal research operations at the station tomorrow. ®