(Reuters) - Space officials in Russia and the United States are tracking hundreds of pieces of debris that were spewed into space when a U.S. satellite collided with a defunct Russian military satellite.

The crash, which Russian officials said took place on Tuesday at about 1700 GMT (12:00 p.m. EST) above northern Siberia, is the first publicly known satellite collision and has raised concerns about the safety of the manned International Space Station.

The collision happened in an orbit heavily used by satellites and other spacecraft and the U.S. Strategic Command, the arm of the Pentagon that handles space, said countries might have to manoeuvre their craft to avoid the debris.

"The collision of these two space apparatuses happened by chance and these two apparatuses have been destroyed," Major-General Alexander Yakushin, first deputy commander of Russia's Space Forces, told Reuters.

"The fragments pose no danger whatsoever to Russian space objects," he said. When asked if the debris posed a danger to other nations' space craft, he said: "As for foreign ones, it is not for me say as it is not in my competency."

The collision between the Iridium Satellite LLC-operated satellite and the Russian Cosmos-2251 military satellite occurred at about 485 miles above the Russian Arctic.

That is an altitude used by satellites that monitor weather, relay communications and perform scientific observations. "It's a very important orbit for a lot of satellites," said Air Force Colonel Les Kodlick from the U.S. Strategic Command. "We believe it's the first time that two satellites have collided in orbit."

The U.S. Joint Space Operations Center was tracking 500 to 600 new pieces of debris, some as small as 4 inches across, in addition to the 18,000 or so other man-made objects it previously catalogued in space, he said.

Russian Space Forces said it was monitoring debris that was spread over altitudes between 500 km (310 miles) and 1300 km (807 miles) above earth. The priority is guarding the International Space Station, which orbits at 220 miles, substantially below the collision altitude. One Russian and two U.S. astronauts are currently aboard the station.

The orbit of the ISS can be changed by controllers from Earth but even a tiny piece of debris can cause significant damage to the space station as it travels at 8 km per second.

"If there is any threat to the ISS then there will be an announcement," one Russian space official said. Another said there was little immediate threat to the station.

The crash has underlined concerns about how crowded the orbit paths around the planet have become in recent decades.

But experts said the chances of such a collision are extremely low and added that leading space powers have been racing to develop new ways to destroy orbiting objects.

"The orbital altitude where the collision took place is among the most crowded in low Earth orbit," Texas-based security consultancy Stratfor said in a research note.

"But statistically speaking, the enormous scale of space makes the chance that this kind of direct collision would occur completely by accident infinitesimal," it said.

The collision occurred in a polar orbit not far from that of a defunct Chinese weather satellite shot apart by a ground-based ballistic missile in a Chinese weapons test in January 2007.

The United States used a missile to blow apart a tank of toxic fuel on a defective U.S. spy satellite last February.

There was no indication that Tuesday's collision was intentional on the part of anyone, said a U.S. government source who asked not to be named.

The European Union said on Thursday leading nations should adopt a code of conduct for civil and military activities in space.