The speeding cloud of debris created when two communication satellites collided this week could pose a risk to other satellites nearby, officials and experts said Thursday.

The collision — believed to be the first of its kind involving two intact spacecraft — occurred 800 km over Siberia on Tuesday and involved a working U.S. commercial satellite launched in 1997 and a derelict Russian satellite launched in 1993 that was believed to be non-functioning.

Iridium Holdings LLC, the Bethesda, Maryland-based company that operates the U.S. satellite and 65 other satellites that relay calls from portable phones, issued a statement Thursday denying responsibility for the crash.

The U.S. Space Surveillance Network and Russian Space Forces are both tracking the debris, which is believed to be traveling at speeds of around 200 metres per second, or 720 km per hour. NASA officials said they don't know how much debris the collision created.

"Right now, they're definitely counting dozens," said Mark Matney, an orbital debris scientist at Johnson Space Center in Houston. "I would suspect that they'll be counting hundreds when the counting is done."

The crash caused space debris to scatter in orbits 500 km to 1,300 km above the Earth, according to Maj.-Gen. Alexander Yakushin, chief of staff for the Russian military's Space Forces.

NASA said there was little risk to the International Space Station and its three-person crew and that the debris posed no threat to the planned launch of the space shuttle Discovery, currently slated for no earlier than Feb. 22.

Canadian satellites potentially at risk

The space station orbits the Earth at an altitude of roughly 320 km, lower than the range of debris created by the collision.

However, other satellites may be at greater risk of collision as a result of the stray debris, experts say.

Canadian Space Society president Kevin Shortt said Canada's Radarsat satellites, which orbit at an altitude of about 800 km above the Earth, are potentially at risk of flying through the debris cloud.

Igor Lisov, a prominent Russian space expert, said there are "quite a lot" of satellites in nearby orbits.

"The other 65 Iridium satellites in similar orbits will face the most serious risk, and there numerous Earth-tracking and weather satellites in nearby orbits. Fragments may trigger a chain of collisions," he said.

Also potentially at risk is the Hubble Space Telescope, according to Nicholas Johnson, an orbital debris expert at the Houston Space Center. The telescope, scheduled to be repaired for one final time in May, orbits the Earth at an altitude of about 560 km.

NASA said there have been other cases in which space objects have collided, but these were minor by comparison and involved parts of spent rockets or small satellites and not two intact spacecrafts.

The Iridium craft weighed 560 kilograms and the Russian craft weighed nearly a tonne.

With files from the Associated Press