According to the Russian space agency Roscosmos, US authorities have refused an entry visa for cosmonaut Nikolai Chub.
"This sets a dangerous precedent for cooperation on the International Space Station," Roscosmos chief Dmitry Rogozin wrote on Telegram on Saturday.
Moscow has always issued visas to US astronauts.
Rogozin asked the US space agency NASA to comment on the case.
He hopes that this is just a misunderstanding, he told the Echo Moskvy radio station.
Joint training actually usual
According to the information, Tschub wanted to prepare for a flight to the ISS planned for 2023 together with US colleagues at the Johnson Space Center in Houston, Texas.
Such training is common.
Russia and the USA want to take astronauts from the other country back to the ISS in their spaceships.
Since NASA has been using private US companies to do this, astronauts and cosmonauts have flown separately to mankind's outpost.
Given the many conflicts between Moscow and Washington, space travel has always been considered one of the few areas where cooperation has worked.
At the turn of the year, NASA agreed to continue operating the ISS until 2030.
Roskosmos now wants to propose a corresponding extension to the government in Moscow.
Last year, Rogozin threatened to withdraw from the project as early as 2025, citing US sanctions against its own space companies.