BAIKONUR, Kazakhstan — NASA's top official said Friday that the future of U.S participation in Russian space flights is in doubt due to a congressional measure that aims to punish Moscow for its cooperation with Iran.

NASA Administrator Michael Griffin told reporters near the Baikonur cosmodrome that "an acceptable financial agreement" could be reached in response to Kremlin demands that the United States pay for its participation in future Russian flights.

But Griffin said the chief obstacle that could end "a continuous American presence on the ISS (international space station)" was the 2000 Iran Nonproliferation Act, which penalizes countries that sell unconventional weapons and missile technology to Iran.

Since the 2003 Columbia disaster, Russia's Soyuz and Progress spacecraft have served as the workhorses of the joint space projects, ferrying crews and cargo to the space station and serving as its lifeline. Discovery visited the station in July, but problems with the foam insulation on its external fuel tank have cast doubt on when the shuttle will fly again. The shuttle can carry vastly greater loads and crews than the Russian spacecraft.

The INA bans U.S. payments to Russia in connection with the space station unless the president determines that Russia is taking steps to prevent transfers to Iran of weapons of mass destruction, missile technology and advanced conventional weapons technology.

Russia is building a $800 million nuclear power plant in Iran despite U.S objections that this could help Tehran to acquire nuclear weapons.