BEIRUT, LEBANON - Secretary of State Hillary Rodham Clinton indicated on Wednesday that Washington is as concerned about Syria's chemical weapons falling into the hands of Syrian rebels as it is about the possibility that President Bashar Assad may decide to deploy them. "Our concerns are that an increasingly desperate Assad regime might turn to chemical weapons or might lose control of them to one of the many groups that are now operating within Syria," Clinton said at the end of two days of NATO meetings in Belgium. "We have sent an unmistakable message that this would cross a red line."

Rebels have been making substantial territorial gains in Syria and have overrun a number of Syrian military bases, capturing supplies of conventional weapons. U.S. officials have indicated that Syria's chemical weapons stocks remain secure, but officials and experts have voiced concern that insurgents could seize facilities where chemical agents are stored.

Western and Israeli officials are believed to be closely monitoring known Syrian chemical weapons depots. An unanswered question is whether signs of a rebel approach on a chemical facility would trigger an international response to prevent toxic weaponry from falling into the hands of insurgent militias.

The fragmented Syrian rebel force includes many Islamist units and several brigades said to be linked to Al-Qaida. The presence of such militants is one reason why the Obama administration has thus far declined to provide weapons to the rebels fighting to oust Assad, whom President Obama has called on to step down.

Assad has taken steps to keep chemical weapons out of the hands of militants, the news agency Reuters reported on Wednesday, citing an interview with Israel's vice prime minister, Moshe Yaalon.

"Clear messages were relayed to Assad on a number of opportunities, and in response Assad in fact gathered up the weaponry and separated the material," Reuters quoted Yaalon as telling the Israeli news website Walla.