US State Department spokeswoman Victoria Nuland said on Monday that the Syrian government "may be organizing another massacre, this time in the village of al-Haffa, in Latakia province, as well as in the towns Deir el-Zour, in Daraa, in Homs, in Hama, and in suburbs of Damascus."

She accused Damascus of using new tactics of repression but made no mention of the armed gangs' failure to abide by the joint UN-Arab League peace plan, brokered by international envoy Kofi Annan.

In 15 months of violence in Syria, the Houla massacre, in which over 100 civilians were killed in the western town on May 25, was the worst incident.

A Syrian government-appointed fact-finding mission has said armed groups carried out the Houla massacre to frame the government and foment sectarian strife.

But anti-government groups say Syrian government forces were the perpetrators of the acts of carnage in Houla.

Annan's six-point plan, effective from mid-April, calls for the establishment of a cease-fire between the government and the opposition and also says humanitarian groups should be allowed to have access to the population, detainees should be released, and a political dialogue should be started.

The unrest in Syria began in March 2011, with demonstrations being held both against and in support of President Assad's government.

The West and the Syrian opposition accuse the government of killing protesters, but Damascus blames "outlaws, saboteurs, and armed terrorist groups" for the unrest, insisting that it is being orchestrated from abroad.