Syria's main opposition group has said it was "surprised" by the UN observer mission's suspension, and called for the monitors to be armed.

"At a time when the regime is committing its worst crimes against the Syrian people, we are surprised by the UN observers' decision to suspend their work, because of what they described as 'an intensification' of violence," the Syrian National Council said in a statement.

The statement said the UN monitors had failed "to pinpoint the source or type of violence in question."

Suspending the mission, the statement added, "serves the criminal regime's interests, and denies the Syrian people the little protection they had."

The SNC reiterated its calls for the UN Security Council to "intervene quickly, and to pass a resolution under Chapter VII [of the UN Charter] to arm the UN monitors, so that they can defend themselves... and ensure that the regime stops killing, while enforcing [UN-Arab League envoy Kofi Annan's] peace plan."

Syrian President Bashar al-Assad's army intensified shelling of Sunni Muslim districts in the northwest city of Homs today, killing at least 11 people and wounding dozens hours after the monitors suspended their work, opposition activists said.

"Around 85% of Homs is now under shelling or bombardment with mortar rounds and heavy machineguns," opposition campaigner Abu Imad told Reuters.

"Dozens of wounded are without treatment because all the hospitals have fallen under the control of shabbiha [ghosts]," he said, referring to militiamen loyal to Mr Assad.

"The dead are the lucky ones."

Reports from Syria cannot be independently verified as the government has banned international media.

Another opposition activist, Mohammad al-Homsi, said: "Since the observers stopped working yesterday we have seen a clear escalation."