UPDATE - Nasa has now said there was "no ammonia leak". Read more here.

Astronauts on board the International Space Station have been evacuated after a toxic ammonia leak.

The leak took place in the US sector of the ship. According to Russia Today, the crew is safe but has been forced to take refuge in the Russian unit.

The contaminated sector has been isolated. Nasa confirmed the leak to NBC and CBS and said it is currently working on a plan to address the issue.

Former ISS commander Chris Hadfield tweeted: "Emergency on the Space Station. High pressure ammonia may be leaking inside. Crew closed hatches, safe for now in Russian segment. Analysing."

The leak took place at 11.44 GMT. Russian press said the atmosphere of the ISS – except the isolated area - remains stable, with the concentration of air pollutants within acceptable levels.

Maksim Matyushin, head of the Russian Mission Control Centre, said: "The safety of the crew has been secured by prompt joint action by the US and Russian crewmembers, as well as by mission control staff in Moscow and Houston."

Ammonia is used on board the ISS to cool the stations power system that provide electricity to the station systems.

A similar incident took place in 2013 when there was a "very serious" ammonia leak. Like the latest event, the leak took place in the US segment, with the Russian side operating as normal.

Vladimir Solovyov, flight director of the Russian segment of the space station, said at the time: "Indeed, they have a serious defect, very serious. It's not the first time such a situation has happened, unfortunately."

Following the 2013 leak, astronauts Chris Cassidy and Tom Marshburn repaired the leak by replacing a pump controller box on the ISS's far port truss that was leaking ammonia. The spacewalk took five-and-a-half hours.

The leak follows the docking of the SpaceX cargo vessel Dragon. The spacecraft docked with the ISS on 12 January and crew began unloading the 5,000lbs of equipment and supplies yesterday.