The US military said today it had detected no launch of a foreign military missile off the coast of California and offered assurances that whatever happened there posed no threat to the United States.

KCBS television caught on camera what appeared to be a missile vapor trail as it arced into the evening sky west of Los Angeles, sparking reports of a possible missile launch.

"At this time, we can confirm that there is no threat to our nation and from all indications this was not a launch by a foreign military," the US Northern Command and the North American Aerospace Defense (NORAD) Command said in a statement,

The Pentagon, however, said it had no explanation for the sighting and was trying to get to the bottom of it.

Colonel David Lapan, a Pentagon spokesman, earlier said none of the military or Department of Defense (DoD) agencies that might have launched a missile "have come up and said they were involved in this."

"So we're still trying to find out what the contrail off the coast of southern California was caused by," he said. "Right now, all indications are that it was not a DoD activity."

ContrailScience.com, a website that debunks conspiracy theories linked to contrails, suggested the sighting was an optical illusion.

It said an approaching aircraft can leave a horizontal vapor trail that looks like a missile shooting vertically from the ground or sea.

What creates the illusion that it is rising from the ground is that the end of the plume is hidden by the curvature of the earth.

Ordinarily, a missile test would involve closure of air space and notifications to mariners of when to stay clear of the area, but none were known to have been made in this case, Lapan said.

He said it was "implausible" that a military exercise would have been conducted so near Los Angeles' busy international airport.

"That's why at this point the operative term is, unexplained," Lapan added.