Foreign Secretary William Hague has told MPs that Syria is "on the edge of civil war".
Speaking in the House of Commons, he said he believed groups linked to Al-Qaeda have committed attacks and condemned "human rights abuses by armed opposition fighters."
ITV News' Washington Correspondent Robert Moore reports:
Hague also repeated his call for the implementation of the Annan peace plan in Parliament, stating 87,000 people had fled to neighbouring countries, and up to half a million are internally displaced within the country.
He said the tactics being used by the Assad regime are "horrifying reminiscent of the Balkans in the 1990s."
And it appeared Hague's fears were being realised on Monday night as the US State Department reported it believed the Syrian government was preparing a massacre in an opposition stronghold.
A spokeswoman for the department said the United States shares the UN envoy Kofi Annan's "deep alarm at reports that the regime may be organising another massacre" in al-Haffeh.
The reports come from UN observers inside the country. The US State Department spokeswoman warned members of the Syrian army they would be held responsible for their crimes:
– US STATE DEPARTMENT SPOKESWOMAN VICTORIA NULAND
The international community can and does learn what units were responsible for crimes against humanity and you will be held responsible for your actions