A brilliant bright light seen streaking across the night sky over the south-western United States was most likely a fireball - a fragment of an asteroid that entered Earth's atmosphere, Nasa has said.

Residents from Phoenix to Las Vegas to southern California's coastal areas reported to local authorities and media outlets that they saw the light move quickly from west to east last night. Many reported the light as bluish-green and others as yellow and orange. Some captured video of the object.

"We can't say 100%," said Don Yeomans, manager of Nasa's near-Earth object programme, "but it's almost certain that the object was a fireball" - or a very bright meteor - "the size of a basketball or baseball that likely disintegrated before it hit the ground."

The bluish-green colour suggests the object had some magnesium or nickel in it, Mr Yeomans said. Orange is usually an indication it is entering Earth's atmosphere at several miles per second, a moderate rate of speed.

"They make an impressive show for such a small object," Mr Yeomans said.

He added that fireball events are much rarer than shooting stars, but they happen on a weekly basis somewhere on Earth, usually over the ocean.

"It's a natural phenomenon and nothing to be concerned about," he said.

Federal Aviation Administration spokesman Ian Gregor confirmed that there were no aircraft incidents reported in the region.