The experience of the US shuttle, to be retired in 2010, highlighted the need for multiple systems to provide backup for the International Space Station (ISS), Griffin told reporters in Paris.

Griffin praised a robot European freighter that carried out a maiden automatic docking with the ISS in April, and said he was deeply enthusiastic about proposals to transform it into a crew transporter.

"I think it's a great idea. I would love to Europe to do that," he said.

"We have moved into an era where the advanced nations of the world in partnership now have substantial space assets at risk.

"We must make sure that we have an adequate logistics capability to support those assets or our investment is lost. So for those reasons I am very much in support of independent Europe access to the station and to other things."

The European Space Agency (ESA), and individual European nations, have never had a manned spacecraft on the grounds of cost and value for money.

Plans to build a three-man mini-shuttle, called Hermes, to be launched on the Ariane-5 rocket, made some headway before being scrapped in the 1990s.

But there remains powerful backing for an independent transporter of some kind, including within ESA itself.

The maiden mission of ESA's Automated Transfer Vehicle (ATV) has spurred the European firm EADS to float a draft design for turning the craft into a crew carrier capable of taking three people.

EADS's Astrium division says the design would mean converting the cargo hold into a capsule and adding a heat shield so that the vehicle survives re-entry, but contends this would not cost too much.

The retirement of the US shuttle, a complex system plagued by two fatal accidents and huge costs to vet it for potential problems at every launch, will leave the ISS completely dependent on Russia's veteran Soyuz system to ferry astronauts to and from the ISS.

Griffin said he was "very concerned" about this dependence, which would probably last from 2010 to 2015, when the new Aries-Orion system -- a rocket-and-capsule concept redolent of the 1960s -- should be ready.

The NASA administrator added he was not perturbed by those voices in the European space community who were urging ESA not to commit too deeply to big-budget US-led prestige projects in the future.

The enormous cost and schedule overruns of the ISS has prompted some space scientists in Europe, as in the United States, to fear for their budgets and to argue that exploration is better served by using robots, rather than humans.

Their concern is that scientific missions will be starved of cash under President George W. Bush's plans to send humans back to the Moon as a stepping stone eventually for going to Mars.

Griffin said that "the hesitation (in Europe) depends on who you talk to," and noted that manned missions had a big camp of European supporters.

"It's a little bit like the old story of the three blind men and the elephant. The picture that you see depends on what part of the elephant you want to touch," he said.

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