A communications satellite belong to US company Iridium hit a disused Russian military satellite on Tuesday, Russian officials said.

The collision took place 800 kilometers (500 miles) above the Earth, Russia's Interfax news agency quoted the head of the country's space forces as saying.

"A collision occurred between an Iridium 33 satellite and a Russian Kosmos 2251 military satellite," Major General Alexander Yakushin said.

The Russian satellite was launched in 1993 and ceased to function two years later, he added.

NASA was tracking hundreds of particles of debris from the collision, and said the orbiting International Space Station faced a small risk of being struck, Space News reported.

In a statement, Iridium called the crash an "extremely unusual, very low-probability event."

Iridium Satellite, which says it has 66 communication satellites plus in-orbit spares, rejected any fault for the accident.

According to Space News, NASA issued an alert Tuesday saying the 900 kilogram (1,980 pound) Russian Cosmos 2251 satellite collided with Iridium's 560-kilogram (1,232-pound) craft at 16:55 GMT, some 790 kilometers (490 miles) above Siberia.

The Washington Post quoted a NASA memo saying officials "have determined that the risk to the space station is elevated, and they estimate the risk to be very small and within acceptable limits."

There is small risk the space station will enter the debris clouds, however, as the ISS is orbiting about 354 kilometers (220 miles) above Earth, some 436 kilometers (270 miles) below the collision orbit.

Cosmic collisions of space junk are not unheard of, but NASA officials said it was the first involving two intact satellites, the Post reported.

NASA spokesman John Yembrick told the paper the collision debris would continue to spread and could end up forcing the space station into evasive maneuvers.

"The space station does have the capability of doing a debris-avoidance maneuver if necessary," and has done so on eight occasions, he said.

Some 6,000 satellites have been sent into space since the Soviet Union launched the first man-made orbiter, Sputnik 1, in 1957. About 3,000 satellites remain in operation, according to NASA.

NASA's space shuttle Discovery is to launch from Florida's Kennedy Space Center on February 22 at the earliest, on a mission to the ISS.

Experts are increasingly concerned about orbital debris, fast accumulating from more than five decades of human activity in space.

Before the latest incident, there were over 300,000 orbital objects measuring between 1 and 10 centimeters (0.4 and four inches) in diameter and "billions" of smaller pieces, according to a report issued last year by an international monitoring group called the Space Security Index.

Travelling at speeds that can reach many thousands of kilometers (miles) per hour, the tiniest debris can damage or destroy a spacecraft worth billions of dollars.

In 1996, a French spy satellite, Cerise, was hit at about 50,000 kilometers (30,000 miles) per hour by a wheeling fragment left from an exploded Ariane rocket.

In June 1983, the windscreen of the US space shuttle Challenger had to be replaced after it was chipped by a fleck of paint measuring 0.3 millimeters (0.01 of an inch), that impacted at four kilometres (2.5 miles) per second.

Space junk eventually falls to Earth, where it is usually completely consumed in the fiery heat of friction with the atmosphere. Re-entry can take weeks, months or many years, depending on how far into orbit the object was.

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