WASHINGTON – Several thousand shoulder-fired missiles – the kind that could be used to shoot down aircraft – are missing in Iraq, and their disappearance has prompted U.S. military and intelligence analysts to increase sharply their estimate of the number of such weapons that may be at large, administration officials said Saturday.

Some U.S. analysts figure that as many as 4,000 surface-to-air missiles once under the control of Saddam Hussein’s government remain unaccounted for. That would raise the number of such missiles outside government hands worldwide to about 6,000.

But a senior defense official said yesterday that military intelligence analysts are having difficulty estimating just how many of the portable missiles may have vanished and how many of those may be in working order and therefore a threat to U.S. and other aircraft.

“We don’t have a good estimate,” the official said. “Some have put forward some figures, but there is none that the Defense Intelligence Agency has confidence in.”

Another official said government analysts could not say with any certainty whether the missing weapons remain in Iraq or have been smuggled outside the country. “There is no evidence that they have left the country,” he said.

Still, other government officials said the threat that the Iraqi missiles could be used to target military or civilian aircraft remains a very real one. Concern about the Iraqi missiles was raised during a conference on aviation threats last week at the DIA’s Missile and Space Intelligence Center in Huntsville, Ala. The new estimates, based on analysis done by the DIA and with the proliferation section of the CIA, were first reported Saturday by the New York Times.

The U.S.-led invasion forces did not secure all weapons depots in Iraq, and hundreds of thousands of tons of munitions were looted. U.S. officials fear that the shoulder-launched missiles were among the items carried off by groups willing to sell them on the black market to terrorist organizations.