Crew members were forced to evacuate the U.S. section of the International Space Station (ISS) on Wednesday following the leak of “harmful substances.” They were moved into the Russia section of the space station, reports Russian Federal Space Agency. NASA confirmed to NBC News that the leaked hazardous substance was ammonia gas.

The incident happened at 4:00 AM EST

The U.S. part of the International Space Station had been sealed off, and all crew were safe. NASA was in the process of coming up with a plan to deal with the situation. A similar ammonia leak occurred on the ISS in May 2013. Astronauts are trained for this, so they responded well. Maxim Matyushin, chief of Russia’s Mission Control Center said that the “concentration of impurities in the atmosphere in the Russian section” of the International Space Station was within permissible levels.

Russian space agency said that the incident happened at around 4:00 AM EST. All the six crew members, including three Russian, two American and one Italian astronaut, are in the Russian part. Russia Today reports that a spacewalk might be required to fix the ammonia leak. The ISS is divided into two parts: the Russian Orbital Segment and the U.S. Orbital Segment.

US Section Of International Space Station Evacuated After Ammonia Leak

International Space Station is a 15-nation space laboratory

The U.S. Orbital Segment is operated by NASA, the Japan Aerospace Exploration Agency, the European Space Agency, and the Canadian Space Agency. The USOS is operated through multiple control centers, including two in the U.S., one in Germany and one in Japan, reports The Independent. The International Space Station is a 15-nation space laboratory overseen by the U.S. and Russia. Due to strained ties between the U.S. and Russia over Ukraine crisis, Russia is planning to discontinue the station’s use beyond 2020.

Last week, a SpaceX rocket delivered about 5,000 pounds of supplies, experiments and equipment at the ISS. The supply mission was delayed twice due to technical issues.