"We don't exactly know the nature of the spill ... but the crew is doing well," said Mike Suffredini, NASA's space station program manager. "It's not a life-threatening material."

In this image from NASA TV, International Space Station astronaut Jeff Williams, left, and European Space Agency astronaut Thomas Retter of Germany, work in the laboratory of the station, Monday, Sept. 18, 2006. [AP]

The crew first reported smoke but it turned out to be an irritant, potassium hydroxide, leaking from an oxygen vent, Suffredini said.

The crew donned surgical gloves and masks but did not have to put on gas or oxygen masks, Suffredini said.

NASA declared a spacecraft emergency for only the second time in the eight-year history of the station. The first time was for a false alarm of an ammonia spill.

NASA initially said that the crew in the orbiting lab 220 miles above Earth had been working on a Russian oxygen-generating system known as the Elektron. But Suffredini said no work on the system had been scheduled at that time.

The Elektron was activated at 6:30 am EDT and shut down about a half hour later. Russian cosmonaut Pavel Vinogradov reported the leak to Mission Control in Russia at 7:23 a.m. EDT.

Vinogradov described the liquid as transparent, "like distilled water."

"At first, small-sized bubbles escaped, drops, four or five," Vinogradov said.

US astronaut Jeff Williams described the smell of burning rubber, but Mission Control in Houston said that odor likely came from the overheating of a rubber gasket.

"That also jibes with the visible smoke coming from the rubber gasket," Williams said.

The station's third crew member is Thomas Reiter of the European Space Agency, who arrived for his six-month stay in July aboard space shuttle Discovery. Williams and Vinogradov are slated to return to Earth at the end of the month.

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