Syrian opposition activists say government forces have intensified their shelling of rebel-held neighborhoods in Homs, killing a least 11 people — 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 that clashes and shelling in several cities 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.”

The escalating violence forced some 300 unarmed United Nations observers to suspend their mission Saturday. But the mission's head says his workers will not leave the country, and on Sunday, a group of U.N. observers travelled from Damascus to Hama to replace staff.

The U.N. sent its observers to Syria to monitor the implementation of a six-point peace plan and cease-fire brokered by U.N. and Arab League Envoy Kofi Annan. But the cease-fire has never taken 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.