Pentagon drafts plans for strike on Syria chemical weapons if 'red line' crossed

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Syrian troops kill 23 rebels in town near Damascus

Steve Harrigan reports from Turkey

The Pentagon has drafted active contingency plans to take out Syria's chemical and biological weapons, sources familiar with the plans told Fox News -- a development that comes as the Obama administration warns of military action if the Assad regime crosses a "red line."

President Obama and now British Prime Minister David Cameron have defined that red line as being when the regime uses, or threatens to use, chemical weapons.

"That would change my calculus. That would change my equation," Obama said Tuesday. Cameron said he agreed, after a phone call with the president Wednesday.

Sources say the Pentagon plans are in place in case Syrian President Bashar Assad takes this step. Fox News confirms the plans would involve using Special Operations teams and air strikes to secure and disable Syria's chemical and biological weapons.

One senior U.S. official told Fox News it would take the Syrians using the weapons, those weapons falling into the wrong hands or a collapse of security surrounding those weapons for the plan to be implemented.

Administration officials have sent stern warnings regarding that potential step, as the death toll continues to rise in Syria along with pressure on the international community to take more aggressive steps to curb the violence.

Activists estimate more than 20,000 have died since the fighting started early last year, with much of the recent bloodshed happening in the city of Aleppo.

After Obama said Tuesday that he's drawing a "red line," his top military official told Fox News that the Pentagon is prepared to act.

Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said the military has a "very professional and a very trained and ready force in the Gulf region ... and it's in a deterrent preparedness posture, but at some point if we're asked to use it, it'll be ready."

As for the president's recent statement, Dempsey said: "I take the president's word that he will consider the use of chemical and biological warfare as a game changer."

Dempsey said U.S. intelligence operatives are focused on monitoring to see if and when the Assad regime is gearing up to use those kinds of weapons.

Fox News' Jennifer Griffin contributed to this report.