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BRUSSELS -- Terrorist groups have expressed interest in obtaining some of the thousands of shoulder-launched missiles that have gone missing in Libya and the issue has become a priority for the Obama administration, a senior U.S. official said Friday.

Andrew J. Shapiro, Assistant Secretary of State for Political-Military Affairs, said Friday the missiles "could pose a threat to civil aviation."

"We know that terrorist groups have expressed interest in obtaining these weapons," he said, adding that the issue issue of securing the weapons was a priority for President Obama and Secretary of State Hillary Clinton.

Libya was believed to have about 20,000 such missiles in its arsenals before civil war began in March, Shapiro said. Although many were destroyed by NATO air strikes, thousands are missing.

"The possibility that these weapons may cross borders is an area of considerable concern," Shapiro said. "That's why U.S. has been working with countries bordering Libya to prevent (proliferation)."

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Reports that thousands of the portable, short-range missiles were missing first surfaced at the end of September, when NATO's top military officer, Adm. Giampaolo Di Paola, was cited as telling German lawmakers that the alliance had lost track of at least 10,000 surface-to-air missiles from Libyan military depots.

The State Department had sent 15 specialists to Libya to track down the weapons and plans to increase the number to 50 soon, Shapiro said, adding the U.S. has allocated $30 million to the effort.

He said vast majority of the missing missiles were Soviet-made SA-7 Strela (Arrow) with infrared homing.

The United States and other Western nations have been trying for decades to reduce the global stock of portable missiles, fearing they could fall into the hands of terrorists. The small, easily concealable SAM-7s are considered obsolete by modern military standards but could pose a threat to civilian airliners or helicopters.

Weighing just 14 kilograms (31 pounds) and only 1.40-meters (4-feet) long, the 1960s-era missile can reach an altitude of over 3,000 meters (10,000 feet).

Thousands have been used in wars in the Middle East, Latin America, Central Asia and former Yugoslavia. Civilian aircraft as well as U.S. and allied warplanes and helicopters have been damaged or downed by the missiles in Afghanistan and Iraq.

"Libya was largest non-producing country holding MANPADS," he said, referring to the weapons by their official designation of Man-Portable Air-Defense Systems.

Also on Friday, NATO Secretary-General Anders Fogh Rasmussen said that although the missing weapons were a matter of concern, "it's not a part of NATO's mandate to deal with that."

He said that according to a U.N. Security Council resolution it was the responsibility of the new authorities in Libya to make sure the stockpiles of weapons are monitored and controlled effectively.

"But I know that individual NATO allies are also engaging with the new authorities to help them fulfill that task." Fogh Rasmussen said in an AP interview.