Syria is believed to maintain stockpiles comprising unknown quantities of biological weapons and hundreds of tons of chemical weapons.

Charles P. Blair, a senior fellow at the Federation of American Scientists who focuses on the issue of terrorism and chemical and biological weapons, warned in a report in March that the dangers posed by Syria’s unconventional weapons are far greater than those posed by Libya’s stockpiles during the 2011 uprising against the late dictator Moammar Gadhafi.

In a grim report for the Bulletin of the Atomic Scientists, Blair wrote that Syria’s chemical warfare stores are believed to be "massive" and include several hundred tons of blistering agents and "likely large stockpiles of deadly nerve agents, including VX, the most toxic of all chemical weapons."

Moreover, at least four large chemical weapon production facilities exist, with storage facilities at dozens of sites throughout the country, he wrote.

Speaking in Prague, Clinton declined to discuss the U.S. response should Assad use chemical weapons.

“This is a red line for the United States,” she said. “I am not going to telegraph in any specifics what we would do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people. But suffice it to say, we are certainly planning to take action if that eventuality were to occur.”

The United States is supporting a request by fellow NATO member Turkey for the alliance to deploy U.S.-made Patriot anti-missile batteries to defend its border with Syria against attack by Syrian missiles tipped with chemical warheads. Foreign ministers of the 28-member pact are expected to approve the deployment at a meeting this week in Brussels.

U.S. officials have been monitoring Syria’s stockpiles since the civil war erupted after Assad’s security forces and gangs of loyalist thugs used force against peaceful protests that erupted in January 2011 calling for an end to four decades of Assad family rule.

More than 40,000 people – most of them civilians – are estimated to have died in the fighting that has leveled whole city blocks and pits rebels mostly comprising Syria’s majority Sunni Muslims against forces and militias led by Assad’s minority Alawite sect, a Shiite Muslim offshoot.

The United States, Israel, Turkey and Arab governments are worried that chemical or biological weapons could fall into the hands of Islamist militants fighting on the rebel side. The extremist groups include Jabhat al Nusra, which has played a substantial role in the rebels’ recent advances, and which U.S. officials call an “al Qaida front.”

"We’re all in agreement that should these weapons pose a threat to start with, or be used, or fall into the wrong hands, this is a game changer," a senior Arab official said Monday, speaking on condition of anonymity, as per diplomatic protocol.

The official declined to discuss whether specific intelligence had led to the latest U.S. warnings to Assad, but he said that the concern was, in part, prompted by the intensified fighting around Damascus.

"The heightened concern is obviously the result of monitoring the situation, obviously as a result of intensification of fighting, obviously as a result of fighting being in and around Damascus now in a concentrated fashion," the official said. "It’s all related."

Lesley Clark and Matthew Schofield of the Washington Bureau contributed.