Opposition activists in Syria say Sunni Muslim districts of Homs have experienced intensified shelling by government forces.
One linked the escalation to the suspension of operations by UN observers.
Another campaigner said 85 percent of the city was under attack.
Several people are said to have been killed on Sunday and dozens wounded. Many reportedly remained untreated as hospitals had fallen under the control of militia loyal to President Bashar al-Assad.
A day after the UN observer mission halted operations, one group of monitors left Damascus for Hama. It has been described as a routine staff deployment.
The head of the mission said on Saturday that increasing violence was jeopardising the observers’ safety and preventing them from carrying out their mandate.
General Robert Mood is expected to brief the United Nations Security council about the violence in Syria early in the week.