By Irene Klotz

CAPE CANAVERAL, Florida (Reuters) - A Russian cargo ship is poised for liftoff on Sunday to the International Space Station following a launch accident in August that forced a cut in staffing levels aboard the orbital outpost.

The Progress freighter, due to launch at 6:11 a.m. EDT from the Baikonur Cosmodrome in Kazakhstan, will be the first flight to the station since an upper-stage rocket failure doomed a similar unmanned freighter on August 24.

The failed motor is nearly identical to those used aboard Russia's Soyuz rockets, which ferry astronauts and cosmonauts to the station, a $100 billion project of 16 nations currently orbiting about 240 miles above the planet.

"The Russians said they had traced the cause (of the accident) to possible debris and clogging in the fuel supply line," said NASA spokesman Kelly Humphries. "They have done additional inspections and testing that they told us about to ensure it's ready to go."

Since the retirement of the U.S. space shuttles this summer, Russian Soyuz capsules are the only ships capable of flying crews to the station, a service that currently costs NASA about $350 million a year.

Flights were suspended after the Progress failure, leaving a half-size, three-member crew aboard the station for an extended period of time.

"Things happen during launches. They are very dramatic and they get everyone's attention," NASA astronaut Mark Polansky, director of operations in Russia, told Reuters.

"What is important is to say, 'OK, there is a problem' and make sure it is fully analyzed and we truly understand what the real root cause is," Polansky said.

RUSSIAN MONOPOLY

If Sunday's launch is successful, a new crew would fly to the outpost on November 13. Their scheduled arrival on November 15 would leave six days before the current crew is due to depart. Their replacements would launch between December 21 and 26.

"It's going to be a fast handover, but they've been making preparations," Humphries said. "In general, they've been doing what they can to document the location of systems and items that crew is going to need."

The preparations include making training videos.

If Sunday's launch fails, the station likely would be left untended after the current crew returns next month. The station has been permanently staffed since the first live-aboard crew arrived on November 2, 2000.

To make room for the new cargo ship, which is scheduled to arrive on Wednesday, station commander Mike Fossum and his crew, Russian cosmonaut Sergei Volkov and Japan's Satoshi Furukawa, packed up an old Progress capsule mounted to the station with trash and other items no longer needed.

The freighter will be autonomously flown away from the station on Saturday and fall back into the atmosphere, where it will be incinerated.

The new cargo ship is filled with 2.8 tons of food, fuel and supplies, including a pair of iPads, which will be used for crew entertainment.

NASA is seeking $850 million this year to help U.S.-based private companies develop space taxis, with the goal of breaking Russia's monopoly on station crew ferry flights before the end of 2016.

"The recent Soyuz failure reminds us that the very existence of the space station is now in jeopardy," George Sowers, vice president of business development for United Launch Alliance, a joint venture of Lockheed Martin Corp and Boeing Co said on Wednesday during a Congressional hearing on NASA's Commercial Crew Program.

"We're reliant on a single, fragile lifeline that we have little insight into or control over."

The United States spent more than 10 years building the station, which is as large as a five-bedroom house, to conduct research and test new technologies in the unique environment of microgravity.

(Additional reporting by Alissa de Carbonnel in Moscow; Editing by Tom Brown and Paul Simao)