The US defence department today said it was trying to determine whether a missile was launched yesterday off the coast of southern California, and who may have launched it.

Spokesmen for the navy, air force, defence department and North American Aerospace Defence Command (Norad) said they were looking into a video posted on the CBS News website. The video appears to show a rocket or some other object shooting up into the sky and leaving a large contrail over the Pacific Ocean.

The video was shot by a KCBS helicopter, the television station said.

Colonel Dave Lapan, speaking for the Pentagon, said: "Nobody within the department of defence that we have reached out to has been able to explain what this contrail is, where it came from. So far, we've come up empty with any explanation."

Lapan said officials were talking to the air force, the navy and Norad as well as to the civilian authorities that control and monitor air space.

"Right now, all indications are that there was not (department of defence) involvement in this," Lapan said, adding that an object could have been launched by a private company.

Officials had no information to make them suspect that the action had been taken by any adversary of the US.

"At this point, until we know more information about what it may have been, there is not alarm," Lapan said. "But that could change, depending on what we find out."