More than 100 people were killed Monday in violence across Syria, as international mediator Kofi Annan said he was "gravely concerned" by the Syrian regime's use of heavy weaponry to attack rebel strongholds and by reports of civilians trapped in besieged cities, DPA reported.
Activists said Monday's death toll reached more than 100, mainly in the central province of Homs, al Heffah in the coastal province of Latakia, as well as Deir al-Zour, east of the country.
The London-based Syrian Observatory for Human Rights reported that a booby trapped car exploded shortly before midnight in Deir al-Zour, killing at least five people and wounding scores of others.
The surge in violence came as Annan, the United Nations-Arab League envoy, expressed worries about the situation in the central province of Homs and al-Heffah in the coastal province of Latakia, his spokesman Ahmad Fawzi said in Geneva.
Both provinces have been under constant, fierce shelling for the past few days. Residents have been appealing for help to the UN observers, who are supervising the fractured ceasefire that went into effect in April.
The United States expressed "deep alarm" over reports that the regime may be organizing massacres in al-Heffah, in the suburbs of Damascus, and towns in Daraa, Homs and Hama, State Department spokeswoman Victoria Nuland said in Washington.
The government's reported use of "new, horrific tactics" including helicopter fire against civilians, was a "a very serious escalation," she said.
Annan has demanded that UN observers be allowed into al-Heffah. He "is gravely concerned by the latest reports of violence coming out of Syria and the escalation of fighting by both government and opposition forces," Fawzi said.
UN observers said many civilians were trapped in the Khalidiyeh neighbourhood of Homs province and that their teams were trying to negotiate evacuations.
"The mission has received reports of a large number of civilians, including women and children, trapped inside the town," the UN Supervision Mission in Syria said in a statement.
It said reports of a large number of casualties could not yet be confirmed.
Activists said Khalidiyeh and two other neighbourhoods were under siege by the Syrian Army.
UN monitors also reported heavy clashes in al-Rastan and Talbisa, north of Homs.
"Artillery and mortar shelling, as well as firing from helicopters, machine guns and smaller arms were being used," the UN statement said. "In Talbisa, UN observers reported that the (opposition) Free Syrian Army captured Syrian Army soldiers."
State media reported that the authorities have told the UN observers they could enter the tense areas, but "on their own responsibility."
Opposition activists have also warned that areas in Homs and al-Heffah might witness killings similar to those in al-Houla and al-Kubair, where scores of people were massacred allegedly by regime forces.
Activists in Latakia described the situation in al-Heffah as "critical and dangerous," and said it was impossible to get information about casualties as the area was surrounded by army tanks.
They said most of the 30,000 residents of al-Heffah had fled, but some stayed back to help the rebels defend the town.
State media reported that "terrorists" in al-Heffah were preparing "for a massacre inside the village." The reports alarmed activists, who called on Annan and the UN observers to intervene "to stop hidden plans" by the government.
British Foreign Secretary William Hague told parliament Monday: "We also have reason to believe that terrorist groups affiliated to al-Qaeda have committed attacks designed to exacerbate the violence, with serious implications for international security." He didn't provide further details.
Hague repeated his view that the situation in Syria increasingly resembled that "in Bosnia in the 1990s."
Nuland of the US State Department recalled the war crimes prosecutions against accused perpetrators from the Balkan wars of the 1990s: "We remind Syrian commanders of one of the lessons from Bosnia. The international community can and does learn what units were responsible for crimes against humanity, and you will be held responsible for your actions."
While Hague warned that there could not be an "open-ended commitment" to Annan's peace plan, he said Britain did not want to see it fail.
"But if, despite our best efforts, it does not succeed, we would have to consider other options for resolving the crisis. In our view all options would then be on the table," Hague said.
According to the Syrian Observatory for Human Rights, more than 14,100 people have been killed since the uprising started in March 2011. The UN estimates more than 9,000 people have died.