The crew of the International Space Station evacuated its U.S. section on Wednesday morning after an alarm sounded, indicating a possible ammonia leak.. (Reuters)

Update: NASA has now confirmed that the leak was a false alarm, and U.S. astronauts will return to their segment of the space station. They'll wear oxygen masks while conducting tests for ammonia, and remove them once they confirm that the air is clear.

A portion of the International Space Station was evacuated following coolant loop pressure increases that possibly indicated a toxic leak of ammonia at 4 a.m. Eastern time Wednesday. All six members of the space station's crew are safe. An afternoon update from NASA confirmed that the problem was not an actual gas leak but a computer error that caused an alarm to sound unnecessarily.

The U.S. side of the space station was sealed off, the Russian Federal Space Agency said on its Web site. The statement characterized the situation as a leak of "harmful substances."

"The safety of the team was preserved thanks to swift actions of the cosmonauts and astronauts themselves and the team on the ground in Moscow and Houston," Maksim Matyushin, head of Russian Mission Control, said in the statement, as translated by NBC News.

The current Expedition 42 crew of the ISS includes three Russians, an Italian and two Americans.

"The crew is safe, they're in the Russian segment, and we're working on understanding exactly what went on,” NASA spokesperson Kelly Humphries told The Post on Wednesday morning. Humphries explained that “we saw an increase in water loop pressure, on the crew cabin pressure, that could be indicative of an ammonia leak in a worst-case scenario.”

Although some reports, citing NASA officials, characterized the issue as an ammonia leak, NASA tweeted Wednesday morning that there is "no ammonia leak confirmed."

This was repeated during an emergency news conference at 7:55 a.m., when NASA representatives said they wished to emphasize that there was "no hard evidence" of a real ammonia leak on the space station. The U.S. crew did indeed don oxygen masks and move into the Russian segment of ISS, shutting down non-essential equipment and shutting the hatch behind them.

But as of that news conference, equipment in the U.S. segment was already being turned back on. Flight control teams on the ground will continue to monitor the data from ISS, which they believe is indicative of a perfect storm of sensor errors — not an actual leak of toxic fluid.

In any case, NASA representatives said during the live update, they didn't see an rapid change in oxygen levels that would put the crew in danger. Even if there had been some kind of ammonia leak, it would have taken a full day before the U.S. segment of the space station reached any kind of dangerous condition.

According to an update from NASA Wednesday afternoon, an alarm indicating an ammonia leak sounded because one of the station's computer relay systems had an error. After a reboot, the relay seemed to be in working condition. NASA flight control reported that the U.S. astronauts had been sent back into their segment.

The astronauts stayed out of the U.S. module until their safety was absolutely certain, so their day (which is usually packed with planned experiments from start to finish) was thrown off schedule. But they were never in any danger. And even in the event of a critical toxic leak, NASA reported, all six astronauts could have lived on provisions in the Russian segment of ISS for more than a week.

As the situation unfolded in space, Canadian astronaut Chris Hadfield — now safely on the ground — tweeted some context on what an ammonia leak would mean for the aging ISS:

The first component of the ISS was launched in 1998, and the space station has had ammonia leak problems before. The last big one, in 2013, was external and required an emergency spacewalk.

Joel Achenbach contributed to this post, which has been updated.