The U.S. and its allies are closely monitoring Syria's stockpiles of chemical arms and portable anti-aircraft missiles, a State Department official says, amid concerns that the country's unconventional weapons could fall into the hands of terrorist or militant groups while the 11-month-old uprising continues, AP reported.

"Syria is a country of significant proliferation concern, so we monitor its chemical weapons activities very closely," the State Department official said, speaking on condition of anonymity to discuss intelligence-related matters. "We believe Syria's chemical weapons stockpile remains under Syrian government control, and we will continue to work closely with like-minded countries to impede proliferation (of) Syria's chemical weapons program."

The official added that the U.S. is in discussion with its allies on ways to ensure that Syria's stockpile of portable anti-aircraft missiles, called Man-Portable Air Defense Systems, or MANPADS, aren't stolen or diverted. "We are consulting with allies and partners as we plan for a variety of contingencies," the official said.

The task of securing Syrian President Bashar Assad's arsenals is complicated by the fact that the U.S. can't be certain it knows how many weapons Syria has and where they are stored. "There's a lot to worry about and oftentimes very little information to assess the situation," said Matthew Schroeder, a small arms researcher with the Federation of American Scientists.

According to independent military experts, Syria had more than 4,000 portable anti-aircraft missiles in the late 2000s. But Syria is a major customer of arms suppliers, including Russia, and could have purchased many more since then.

However, Syria's chemical arms are believed to be secure for now because they are stored at weapons depots in rural areas, officials and experts say, away from the urban centers where most fighting is now taking place.