Russia's space agency reported an unexplained loss of pressure on Saturday in a cargo craft docked at the International Space Station 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 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.
The initial statement from Roscosmos left it unclear whether the entire cargo ship or just some of its systems lost pressure, but Sergei Krikalev, head of Roscosmos’ crewed programmes, later clarified that there was depressurisation of the craft’s coolant loop.
It is the latest glitch to affect the ageing ISS after a coolant leak in December on a Soyuz MS-22 spacecraft docked to the station. Russian space officials said it had been caused by a tiny meteoroid that left a small hole in the exterior radiator and sent coolant spewing into space.
Roscosmos has launched a probe into a possible cause of the cooling loop leak, and Mr Krikalev said that experts will closely look at materials and technologies used in spacecraft manufacturing as part of the probe.
“We need to conduct a thorough analysis to make sure that it wouldn't affect similar components that will be used in the future,” Mr Krikalev said. “This is the most important task.”
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 US still collaborate closely on the ISS despite the damage to their relations from Moscow's invasion of Ukraine. There are currently seven crew on board — three Americans, three Russians and one Japanese.