Ban Ki Moon, the UN secretary-general, has said he is "gravely alarmed" by reports of shelling in populated areas in Syria, after the death of scores of people in the city of Hama.
In a statement released on Thursday, Ban said the Syrian government was "in contravention" of an agreed peace plan by keeping troops and heavy weapons in cities.
On Wednesday an explosion destroyed a building in Syria's central city of Hama, reportedly killing up to 69 people, many of them children.
The UN chief condemned the government of President Bashar al-Assad for using excessive force against the Syrian civilian population, calling the situation as "unacceptable".
He demanded that Assad's government must "comply with its commitments without delay."
The statement comes after Kofi Annan, the joint UN- Arab League special envoy to Syria, told the Security Council on Tuesday that the country was still experiencing unacceptable levels of violence, despite an April 12 ceasefire based on his six-point plan.
Syrian authorities say they are committed to Annan's April 12 ceasefire agreement, but reserve the right to respond to what they say are continued attacks by "terrorist groups".
The UN Security Council unanimously adopted a resolution on Saturday, authorising an initial deployment of up to 300 unarmed military observers to Syria for three months to monitor the cease-fire.
Only 15 are in place so far out of a team to be led by Norwegian General Robert Mood.
On Thursday, The Arab League said it would ask the UN Security Council to ``review'' its policy on Syria if the regime there fails to fully and immediately honour its commitment to a cease-fire.
Arab foreign ministers said in a statement released after their meeting in Cairo on Thursday that the Syrian regime was negotiating while simultaneously "killing its own people."
It said Morocco, currently a member of the 15-nation UN Security Council, to convey the League's request when the world body meets to discuss Syria on May 5.