In tit-for-tat move, Russia denies visa to NASA envoy

"It was a retaliatory measure," Russian Deputy Foreign Minister Sergei Ryabkov told the state-run RIA Novosti news agency.

Citing sources in the space sector, RIA Novosti reported that Russia in January denied the visa to a NASA member who was set to head the agency's office at the US embassy in Moscow.

The sources did not name the NASA employee in question.

Russia has for months complained that American authorities have blocked visas to its diplomats, or issued them slowly.

Tensions between Moscow and Washington have reached their lowest point since the end of the Cold War following allegations of hacking and election interference.

But space has remained one of the few areas of cooperation between the two countries, dating back to the Soviet era.

While Ryabkov admitted that NASA was not at fault for Russia's visa difficulties, he said that "this has often taken place" with other US agencies and called for the lifting of these "artificial obstacles" in US-Russia relations.

Although Moscow and Washington recently extended a landmark nuclear accord, the New START treaty, a number of disagreements remain between the former Cold War rivals.