WASHINGTON - US space agency NASA has suspended all contacts with its Russian counterpart Roscosmos except for ongoing activities aboard the International Space Station (ISS), according to an internal letter published by the US space news website Spaceref on Wednesday, citing US-Russian tensions over Ukraine and Crimea.

"This suspension includes NASA travel to Russia and visits by Russian Government representatives to NASA facilities, bilateral meetings, email, and teleconferences or videoconferences," the letter said.

"At the present time, only operational International Space Station activities have been excepted," it said.

There are currently two Americans onboard the ISS.

In addition, multilateral meetings held outside of Russia that may include Russian participation are not precluded under the present guidance, the letter added.

In March, NASA Administrator Charles Bolden has repeatedly said that space cooperation between the two countries would not be affected over tensions in Ukraine.