Russia's contract to ferry NASA astronauts to the International Space Station aboard Soyuz rockets will end in April, Deputy Prime Minister Yuri Borisov told reporters on Friday.

The expiration piles additional pressure on the National Aeronautics and Space Administration to restore its capability to shuttle U.S. crew members back and forth to the orbiting lab. The space agency is contracting with Boeing Co. and SpaceX to develop new vehicles to transport astronauts, but the work has been plagued by delays.

NASA has relied on Russia since retirement of the space shuttle in 2011 ended U.S.-controlled access to the space station. Congress and President Donald Trump's administration have touted the commercial program's importance to ending that reliance, especially as diplomatic relations between the nations have deteriorated.