Russia's space agency reported an unexplained loss of pressure on Saturday in the Progress MS-21 cargo craft docked to the International Space Station (ISS) but said there was no threat to the crew.

Roscosmos said mission control specialists had detected a depressurisation in the cooling system of the Progress MS-21 cargo ship which is due to undock from the ISS on February 18.

The vessel was sealed off from the rest of the station and the reason was being investigated, it said.

"The temperature regime and pressure on board the ISS are normal, nothing threatens the life and health of the crew and they feel fine," Roscosmos said.

Aging ISS

It was the latest glitch to affect the aging ISS after a coolant leak in December on a Soyuz MS-22 spacecraft docked to the station.

NASA said its specialists were helping their Russian counterparts in troubleshooting the latest leak.

"Officials are monitoring all International Space Station systems and are not tracking any other issues," it said in a blog post.

The Soyuz craft are used to ferry crew to and from the station, while the Progress vessels deliver equipment and supplies. Another Progress craft, MS-22, docked successfully with the ISS earlier on Saturday.

The problem with the Soyuz has forced three of the current crew on the station, Russians Sergey Prokopyev and Dmitry Petelin and NASA astronaut Frank Rubio, to extend their mission. They will now return to Earth on a replacement Soyuz that Russia is due to launch on February 20.

Russia and the U.S still collaborate closely on the ISS despite the grave damage to their relations from Russia's invasion of Ukraine. At the moment there are seven crew on board, three Americans, three Russians and one Japanese.

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(With input from Reuters)