WASHINGTON -- The difficulties and delays in getting the Russian Zvezda service module launched are raising new concerns about Russia's ability to deliver additional spacecraft to ferry crews and cargo to the nascent International Space Station (ISS).

Even if Zvezda (Russian for "star") can be safely launched and docked with the ISS, Moscow remains firmly in the "critical path" of the station's construction because of NASA's need for a steady stream of Russian supply craft.

Without those craft, it would be difficult -- if not impossible -- to put live-aboard crews on the ISS.

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NASA is counting on the Russians to make good on a promise to deliver each year at least three to four Soyuz capsules, each capable of holding three people, and three to six pilotless Progress supply rockets.

"It's something we're watching very closely," said Kirk Shireman, NASA's station manager for Russian components. "It's obviously important not only to the Russians but also the U.S and our other partners. We're confident they can meet those obligations."

Others are not so sure, pointing to financial difficulties in Russia that could derail plans to provide Soyuz and Progress spacecraft to the ISS.

"It's an open issue," said Charles Vick, a space analyst at the Federation of American Scientists in Washington.

"The Russians say they are going to provide these things and NASA's been looking over everybody's necks to keep them working on them. But from the reports I've seen, I doubt if NASA is very certain the Russians can do it," Vick said.

The Russians now have 20 Soyuz and Progress vehicles in production, NASA says.

The Soyuz, Russia's only piloted launch vehicle, was designed in the 1960s for trips around the moon. At a length of 29 feet (9 meters) and weighing about 14,520 pounds (6,600 kilograms), it consists of a near-spherical orbital module, a bell-shaped reentry unit and a service module.

The Soyuz would be used to ferry crews to the ISS -- just as it has done at the Russian Mir space station -- and would serve as an emergency "lifeboat" should the station need to be evacuated. NASA's strict rules require that a Soyuz be docked at all times to an inhabited ISS.

The pilotless Progress, a modified version of the Soyuz, is used to deliver fuel and dry goods to a space station via remote control. It can carry about 4,000 pounds (1,800 kilograms) of cargo. After emptying the Progress, station crews typically fill it with garbage before jettisoning it from the station to burn up in Earth's atmosphere.

What cash there is for Russia's space program has to be balanced between two space stations: the ISS and the aging Mir. It's a dilemma that continues to weigh heavy on the minds of NASA planners, who turned to the Russians for their expertise in long-duration spaceflight.

"As has always been the case, the question is how they want to spend their money," said Marcia Smith, an expert on the Russian program with the Congressional Research Service. "The reason people are concerned is because Mir is still up there and they see the ISS as a competitor with Mir."

Historically, the Russian government has not been willing to commit to the levels of funding needed for the ISS, Smith said.

That may change under Russia's new president, Vladimir Putin, but only if Mir is able to survive on a strictly commercial basis and does not require further government funding.

Putin's statements about space have echoed those of his predecessor, Boris Yeltsin, in that he supports both Mir and the ISS.

"Whether that translates into dollars [for the ISS] remains to be seen," Smith said. "That uncertainty will continue."

NASA believes it has a workable fallback plan in case the Russians aren't able to churn out enough Soyuz and Progress stock.

In the early years, the ISS could get by each year with just three Progress vehicles, each filled with fuel for the Zvezda's thruster rockets. NASA's space shuttles then could haul dry goods like food, clothing and other supplies to the station.

In that scenario, the shuttle also could be used to periodically boost the station to a higher orbit to keep it from falling to Earth.

In later years however, NASA will need six Progress vehicles a year because the mass of the ISS will grow as more modules are added and the shuttle alone won't be able to reboost the station.

Without Soyuz spacecraft, though, NASA could not keep anyone aboard the ISS unless a shuttle was docked at the station.

NASA will eventually have its own crew return vehicle capable of carrying up to seven people -- more than two Soyuz craft -- but that vehicle is not scheduled to be finished until 2005.