MOSCOW (AP) — Russia’s space agency said Wednesday that two cosmonauts scheduled to launch to the International Space Station will be replaced with alternates for medical reasons.

Roscosmos said crew members Nikolai Tikhonov and Andrei Babkin will be replaced by their designated backups, Anatoly Ivanishin and Ivan Vagner, for the launch scheduled for April 9.

Speaking at Johnson Space Center in Houston, NASA spokesman Rob Navias said U.S. astronaut Chris Cassidy, who had trained with Tikhonov and Babkin, would remain on the crew and launch as planned with the two Russian alternates.

Sergei Krikalev, head of manned programs for Roscomos, said in remarks carried by Russian news agencies that the Russian crew members will be replaced due to unspecified medical reasons.

One of the cosmonauts was “temporarily unfit” for the mission, Krikalev said. He refused to elaborate.

Russia’s RIA Novosti news agency said Tikhonov received an unspecified injury during training.

The International Space Station is currently operated by NASA astronauts Jessica Meir and Andrew Morgan and Russian Oleg Skripochka.