NASA's top official said Friday that the future of U.S. participation in Russian space flights was in doubt.

Michael Griffin told reporters near the Baikonur cosmodrome that "an acceptable financial agreement" could be reached to resolve Russian demands that the U.S. pay for its participation in future Russian flights.

But Griffin said a U.S. law the Iran Nonproliferation Act of 2000 penalizing countries that sell unconventional weapons and missile technology to Iran, including Russia could mean an end to "a continuous American presence on the ISS (International Space Station)."

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