WASHINGTON, Dec 28 (AFP) - NASA is exploring whether SpaceX's Crew Dragon spacecraft can potentially offer an alternative ride home for some crew members of the International Space Station after a Russian capsule sprang a coolant leak while docked to the orbital lab.

NASA and Russia's space agency, Roscosmos, are investigating the cause of a punctured coolant line on an external radiator of Russia's Soyuz MS-22 spacecraft, which is supposed to return its crew of two cosmonauts and one US astronaut to Earth early next year.

But the Dec 14 leak, which emptied the Soyuz of a vital fluid used to regulate crew cabin temperatures, has derailed Russia's space station routines, with engineers in Moscow examining whether to launch another Soyuz to retrieve the three-man team that flew to ISS aboard the crippled MS-22 craft.

If Russia cannot launch another Soyuz ship, or decides for some reason that doing so would be too risky, NASA is weighing another option.

"We have asked SpaceX a few questions on their capability to return additional crew members on Dragon if necessary, but that is not our prime focus at this time," NASA spokeswoman Sandra Jones said in a statement to Reuters.

SpaceX did not respond to a Reuters request for comment.

It was unclear what NASA specifically asked of SpaceX's Crew Dragon capabilities, such as whether the company can find a way to increase the crew capacity of the Dragon currently docked to the station, or launch an empty capsule for the crew's rescue.

But the company's potential involvement in a mission led by Russia underscores the degree of precaution NASA is taking to ensure its astronauts can safely return to Earth, should one of the other contingency plans arranged by Russia fall through.

The leaky Soyuz capsule ferried US astronaut Frank Rubio and cosmonauts Sergey Prokopyev and Dimitri Petelin to the space station in September for a six-month mission. They were scheduled to return to Earth in March 2023.

The station's four other crew members - two more from NASA, a third Russian cosmonaut and a Japanese astronaut - arrived in October via a NASA-contracted SpaceX Crew Dragon capsule, which also remains parked at the ISS.

SpaceX's Crew Dragon capsule, a gumdrop-shaped pod with four astronaut seats, has become the centrepiece to NASA's human spaceflight efforts in low-Earth orbit. Besides Russia's Soyuz programme, it is the only entity capable of ferrying humans to the space station and back.