A U.S. government arms expert and several private American weapons disposal contractors are aiding Libyan officials in the hunt for dangerous mobile anti-aircraft systems amid news reports that massive arms caches in Tripoli were looted after the ouster of the Moammar Gadhafi regime, the State Department confirmed Thursday.

Despite the newly-acknowledged presence of American weapons experts, the growing evidence that high-value arms and munitions already have been plundered raises the stakes for the U.S. and Western powers to aid Libya's infant government in quickly finding and securing remaining weapons caches.

State Department spokeswoman Victoria Nuland said news reports of unguarded and emptied weapons depots in Tripoli were "concerning." The issue was front and center in discussions last week in Paris between Secretary of State Hillary Rodham Clinton and officials of Libya's Transitional National Council.

"The secretary sought and got reassurances from the TNC that our work with them will continue," Nuland said. "So that's why we deployed extra personnel. And we're working on it hard now."

Time and the difficulty of finding the Gadhafi regime's hidden storehouses are working against them. The Associated Press and other news media reported Thursday that crates of Russian-built anti-aircraft missiles and other munitions were systematically looted. Emptied crates found in several Tripoli caches by reporters and officials of Human Rights Watch appeared to have contained scores of Russian-built Man-Portable Air Defense Systems, or MANPADS.

Weapons and humanitarian experts have warned for months that plundered anti-aircraft systems could be used by terrorist groups to target unprotected civilian aircraft. U.S. officials estimate the Gadhafi regime hoarded as many as 25,000 such weapons in recent years.

Many of the systems were reportedly older Russian SA-7 shouldered-fired units that date back to the 1970s, and some could be too old to operate. But arms experts have seen evidence that the Libyan regime also had amassed newer Russian SA-24 models that are typically mounted on vehicles and have a longer range in targeting aircraft. In both cases, arms experts say, Libyan officials and Western governments have only a limited amount of time before such weapons vanish.

"They should be moving on this right now," said Matthew Schroeder, a weapons expert with the Federation of American Scientists in Washington. "Obviously the prime concern is making sure these systems stay out of the hands of terrorists. But the U.S. experience in Iraq should also raise warning flags about keeping all sorts of weapons out of any future internal strife."

In the aftermath of the U.S. invasion of Iraq in 2003, arms depots across the country were plundered of arms and explosive materials. Critics later contended some of those munitions were used in deadly attacks and bombings aimed at U.S. troops and Iraqi civilians.

Nuland did not identify the State arms expert or private contractors now in Libya, but the department already had budgeted $3 million to hire two international weapons disposal teams to locate and destroy MANPADS, land mines and other munitions in Libya. Those teams have only destroyed small numbers of MANPADs. Confined so far to western Libyan battle sites, the international teams have yet to reach Tripoli, where many of the new arms caches turned up.

Arms experts also have raised questions about stockpiles of mustard agents and other chemicals amassed by the Gadhafi regime and about radioactive materials stored at a small nuclear reactor facility. U.S. officials have stressed repeatedly that those materials are in safe hands, and Defense Department spokesman made those assurances again Thursday.

Pentagon press secretary George Little said the Obama administration was confident Libya's chemical weapons stockpiles were secure, although he would not explain how the U.S. is so certain.

U.S. officials had said earlier that airborne drones were being used to monitor Libya's borders.

"We have strong confidence that those weapons caches have not moved," Little said.


AP National Security Writer Robert Burns contributed to this report.