The head of the US space agency Nasa has said "we are at the moment of truth" regarding Russia's commitment to the International Space Station (ISS).

In a significant souring of relations between Nasa and Russia, Mr Goldin said he is also summoning its international partners for an "emergency meeting" at the end of February to discuss the role Russia will have in the future of the ISS.

"To say we are frustrated and disappointed is an understatement," an angry Mr Goldin told reporters. "The Russians have got to understand what the focus is here, and that is their commitment to the station. It is up to the Russians to demonstrate that."

Booster problems

The 18-month delay in the launch of Zvezda is partly due to booster problems and partly a lack of confidence that the Russians can launch it when they say they can. Nasa now says it will launch its own "back-up" module that it had built for just this eventuality.

Mr Goldin said that the $200 million US component called the Interim Control Module (ICM) will be launched to the ISS in December if the Russians are unable to put up Zvezda by July.

He added that if Zvezda was put into space in July then the ICM would not need to be launched until 2001. He also said that Nasa will launch a separate $100 million propulsion module that can reboost the ISS and keep it safely in orbit. It is the first time that Nasa has said it will launch both its "back-up" components.

Mr Goldin was also angry at the news that Mir is to receive a new crew in March. He said: "It is not up to America to tell the Russians how to operate Mir," but it called into question their commitment to the ISS. "In no way is the operation of Mir to interfere with their commitments to the International Space Station. And we are at the moment of truth.

"We want the Russians to be on the station with us but I feel we have to be responsible custodians," he said. "We are waiting for a signal that they are really committed" to the project.