NASA has cut ties with Russia except for cooperation aboard the International Space Station due to the crisis in Ukraine, the US space agency said. "Given Russia's ongoing violation of Ukraine's sovereignty and territorial integrity, NASA is suspending the majority of its ongoing engagements with the Russian Federation," said a NASA statement. "NASA and Roscosmos will, however, continue to work together to maintain safe and continuous operation of the International Space Station." The story was first reported by news site The Verge, which obtained an internal memo describing the policy change. A copy of the memo posted online described a halt to travel to Russia by NASA employees and of visits by Russians to NASA facilities, and a freeze on exchange of email, teleconferences and video conferences. The new policy does not apply to "operational International Space Station activities" or "multilateral meetings held outside of Russia that may include Russian participation," said the memo. Russia's Soyuz spacecraft are the sole means of transport to the ISS for the world's astronauts. The United States pays Russia an average of $70.7 million per seat. NASA Administrator Charles Bolden told lawmakers on March 27 that he was aware of no threat that would jeopardize the Russian-US partnership at the space station. Earlier Wednesday, NATO warned that Russia's military presence on the flashpoint border -- estimated at about 40,000 troops -- with Ukraine was of "grave concern." NATO on Tuesday announced the alliance was suspending all practical cooperation with Russia, military and civilian, and that there was no confirmation that Russian troops were pulling back from the border.