The photos feature three Russian cosmonauts holding aloft the flags of two regions of eastern Ukraine which had been captured by Russian military forces - prompting the US space agency to issue a "strong rebuke".

The stunt was described as "fundamentally inconsistent with the station's primary function among the 15 international participating countries to advance science and develop technology for peaceful purposes" by NASA.

Despite the growing terrestrial conflict between Washington and Moscow, co-operation in low-Earth orbit has largely continued with little challenge from the US to its Russian partners - although Roscosmos' chief executive has repeatedly threatened to withdraw co-operation.

Roscosmos on Monday posted the photographs of cosmonauts lifting the flags of the self-declared Luhansk People's Republic and the Donetsk People's Republic which are not recognised by the international community.

There are concerns that diplomatic fallout over the war in Ukraine could undermine the international co-operation necessary to keep the ISS in orbit and astronauts safe.

NASA previously told Sky News that despite heated exchanges and deteriorating relations back on Earth, co-operation between Russia and the US on the ISS will continue.

"There really are no tensions in the team," said Joel Montalbano, programme manager for the ISS.

His comments followed a tongue-in-cheek video posted on social media by Russian government-controlled RIA Novosti, showing NASA astronaut Mark T Vande Hei being left behind on the space station by cosmonauts.

Concerns grew when the video was retweeted by the head of Russian space agency Roscosmos, Dmitry Rogozin.

It was just one of several barbed tweets sent by the Russian space chief aimed at US and European colleagues since sanctions were imposed on Russia.

The end of the ISS

Regardless of the outcome of the invasion of Ukraine and the relations between the US and Russia, the long-term future of the ISS is likely limited.

NASA has published plans which could see the 444,615kg structure taken out of orbit in January 2031 and crashed into a "spacecraft cemetery" in the remotest spot on Earth.

The Commercial Crew Programme is part of NASA's efforts to help the private sector get a foothold in space, ultimately replacing the orbiting laboratory with a number of commercial space stations.

In the perfect scenario, the space station's orbiting altitude will be slowly lowered from its current altitude of 408km (253 miles).

As the altitude of the ISS drops it will encounter an ever-denser atmosphere, adding more drag and pulling it lower still.

The space station will still be travelling so fast that it will begin to heat and cast off debris in a path behind it.

The plan to avoid this debris damaging people or property is to have the ISS crash into an uninhabited area of the south Pacific Ocean, near to Point Nemo.

Point Nemo has been called a spacecraft cemetery because - as the point on the Earth most distant from any land - it is where decommissioned spacecraft are typically aimed when returning to the Earth.