British Foreign Secretary William Hague said on Saturday said that there is evidence that the Syrian government could use its chemical weapons stocks in its conflict with rebels fighting to oust it.

"We are extremely concerned about the stockpiles of chemical and biological weapons, and we are also concerned about evidence during the last couple of weeks that the regime could use them," Hague told reporters in Manama on the sidelines of a regional security conference.

He said Britain had joined with the United States in delivering a strong message to President Bashar al-Assad's government.

"We have contingency plans concerning chemical weapons but will not disclose them," he added.

Hague said there were several "dangerous scenarios" for such weapons, including their "use by the regime, or falling in the hand of other people".

He said the option of military intervention in Syria had not been "ruled out" but Britain continued to support a peaceful transition.

He said it was not Britain's policy "to supply arms to any side in the Middle East" but added that the opposition fighters were "receiving arms and making progress on the ground".

"We will continue to give them strong practical assistance -- communication equipment and humanitarian assistance," the British minister said.

"I hope that the international community will be giving more support to the (opposition) coalition during the Marrakesh meeting," he said, referring to a meeting of the Friends of Syria group due to be held in the southern Moroccan city on December 12.

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."

Washington has said the use of chemical weapons would be a red line but that it fears battlefield advances by the rebels could prompt the Assad regime to use them, or that stocks could fall into the hands of groups hostile to the United States and its allies.

The Syrian government has insisted it would never resort to the use of chemical weapons in the 21-month conflict, which has killed more than 42,000 people, according to the Britain-based Syrian Observatory for Human Rights.