The head of the United Nations observers' mission in Syria has demanded government and rebel forces take immediate action to allow the evacuation of women, children, the elderly and the injured from the besieged city of Homs and other conflict zones.

Major General Robert Mood said Sunday that neither of the warring parties has been willing to hold their fire and that U.N. efforts to evacuate civilians from Homs over the past week have failed.

Opposition activists said government forces have intensified their shelling of Sunni Muslim districts in Homs, killing a least 11 people and wounding dozens one day after U.N. observers suspended their work.

Activists released new amateur video Sunday of what appeared to be heavy shelling in the central Syrian city. The opposition also reported clashes and shelling in several areas killed at least 50 people Saturday.

While it is not possible to independently verify the claims and video, Homs has been the epicenter of the anti-government unrest that has paralyzed Syria for more than a year, killing thousands. The Syrian government blames the violence on what it calls “terrorists.”

Free Syrian Army rebels are holed up with civilians still in Homs after tens of thousands fled over the last year.

The escalating violence forced some 300 unarmed U.N. observers to suspend their mission Saturday. But Mood said his staff will not leave the country, and on Sunday, a group of U.N. observers travelled from Damascus to Hama to replace their colleagues there.

The U.N. sent its observers to Syria to monitor the implementation of a six-point peace plan and cease-fire brokered by international envoy Kofi Annan. But the cease-fire never took hold and several days ago a group of observers came under attack when it went to visit the town of al-Haffeh in Latakiya.

Syrian opposition officials in Istanbul, including the Syrian National Council's Bassam Imadi, said recently they believe the U.N. mission has failed.

U.N. observer teams have also been targets of roadside bombs. Although no one was killed or seriously wounded, U.S. Ambassador to the U.N., Susan Rice, said the observers were “one [explosion] away from a disaster.”

The U.N. Security Council must decide whether to renew the mission by July 15.