By Elise Labott reporting from Manama, Bahrain
British Foreign Secretary William Hague cited evidence that the Syrian regime could use its stockpile of chemical weapons against rebels battling government forces.
"We are extremely concerned," Hague told reporters on the sidelines of a regional security conference Saturday.
Hague said that there was no simple "red line" which could trigger international military action, but that Britain and its allies had "contingency plans concerning chemical weapons" which he declined to disclose.
He noted several "dangerous scenarios" for the use of chemical weapons. One would be terror groups obtaining such munitions.
Recent U.S. intelligence suggests the Syrian government has started mixing chemical weapons compounds and loading them into bombs, though the bombs are not being moved to any delivery devices, CNN's Barbara Starr reported.
The U.S. military continues to revise its plans for a potential strike against Syria over chemical weapons.
President Barack Obama said this summer that any effort to move or use chemical weapons was a red line.
"A red line for us is we start seeing a whole bunch of weapons moving around or being utilized," Obama said. He prefaced that remark by saying the concern was the weapons falling into the hands of others.
With the new intelligence, administration warnings have focused not on moving, but on using the weapons.
As diplomats continue to seek a more peaceful resolution, international powers are weighing their military options.
The Republican chairman of the House Intelligence Committee on Friday warned the Obama administration it must act more urgently to prevent Syria's government from using chemical weapons.
Rep. Mike Rogers told attendees at the IISS Manama Dialogue conference in Bahrain that the United States has a moral obligation to use military intervention if there is concrete proof chemical weapons are loaded and being readied for launch.
"I don't see any other way of making sure those weapons aren't used," he said. "As a coalition, we will have the moral obligation (to intervene) if we can say with even a moderate degree of certainty that these weapons have been prepared and are put in an arsenal for use."
On Friday, UN chief Ban Ki-moon said any use of chemical weapons by President Bashar al-Assad's regime against the rebels would be an "outrageous crime."
Although Hague said the option of international military intervention had not been ruled out, Britain was working along with the United States and others to support a peaceful transition.
Hague acknowledged that rebel fighters were "receiving arms and making progress on the ground," but said Britain's policy was not to send arms to the Middle East.
"We will continue to give them strong practical assistance; communication equipment and humanitarian assistance," he said, adding that he hoped the international community would increase its support to the new Syrian opposition coalition during the Friends of Syria meeting in Marrakesh next week.