Hague cited the Syrian regime's stockpile of chemical weapons, CNN reported.
"We are extremely concerned," Hague said, adding that Britain and its allies had "contingency plans concerning chemical weapons."
He did not elaborate.
The U.S. military has been revising its plans for a potential strike against Syria over chemical weapons.
"A red line for us is we start seeing a whole bunch of weapons moving around or being utilized," President Barack Obama said during a news conference in August.
The U.S. State Department and the White House warned Syrian President Bashar Assad not to use chemical weapons on Syrians.
White House spokesman Jay Carney said Monday "we are concerned that in an increasingly beleaguered regime, having found its escalation of violence through conventional means inadequate, might be considering the use of chemical weapons against the Syrian people."
"The Assad regime must know that the world is watching, and that they will be held accountable by the United States and the international community if they use chemical weapons, or fail to meet their obligations to secure them," Carney said.