The European Space Agency has begun preliminary technical discussions with Elon Musk’s SpaceX that could lead to the temporary use of its launchers after the Ukraine conflict blocked Western access to Russia’s Soyuz rockets.

The private American competitor to Europe’s Arianespace has emerged as a key contender to plug a temporary gap alongside Japan and India, but final decisions depend on the still unresolved timetable for Europe’s delayed Ariane 6 rocket.

“I would say there are two and a half options that we’re discussing. One is SpaceX that is clear. Another one is possibly Japan,” ESA Director General Josef Aschbacher told Reuters.

“Japan is waiting for the inaugural flight of its next generation rocket. Another option could be India,” he added in an interview.

“SpaceX I would say is the more operational of those and certainly one of the back-up launches we are looking at.”

Aschbacher said talks remained at an exploratory phase.

“We of course need to make sure that they are suitable. It’s not like jumping on a bus,” he said. For example, the interface between satellite and launcher must be suitable and the payload must not be compromised by unfamiliar types of launch vibration.

“We are looking into this technical compatibility but we have not asked for a commercial offer yet. We just want to make sure that it would be an option in order to make a decision on asking for a firm commercial offer,” Aschbacher said.

SpaceX did not reply to a request for comment.

SpaceX’s Falcon 9 has already swept up other customers severing ties with Moscow’s increasingly isolated space sector amid the Ukraine conflict, but a high-profile European mission could be seen as a significant win for the US rocket maker.

Aschbacher stressed any back-up solution would be temporary, however, adding he was not worried about the future of Ariane 6.

Satellite internet firm OneWeb, a competitor to SpaceX’s Starlink satellite internet venture, booked at least one Falcon 9 launch in March. It has also booked an Indian launch.

On Monday, Northrop Grumman booked three Falcon 9 missions to ferry NASA cargo to the International Space Station while it designs a new version of its Antares rocket, whose Russian-made engines were withdrawn by Moscow in response to sanctions.