A defunct Russian communications satellite has smashed into a U.S. satellite in orbit, creating a possible risk to the International Space Station.

U.S. officials say this is the first time two whole satellites ever crashed into each other in space.

They collided Tuesday about 780 kilometers above Siberia, creating a huge explosion with many pieces of debris.

The U.S. space agency, NASA, said the floating satellite parts create a small risk to the International Space Station, which flies at a lower orbit than where the collision took place.

But NASA says it will be weeks before the full magnitude of the collision is known.

Scientists say there are thousands of pieces of space junk orbiting the Earth, including old satellites and burned-out rocket boosters.

Some information for this report was provided by AP and Reuters.