BEIRUT - Syria said on Tuesday it was trying to evacuate civilians from the city of Homs and blamed rebel fighters for obstructing efforts to get people out safely.

Activists said the army has intensified shelling of residential districts of Homs, center of the revolt against President Bashar Assad, and the head of United Nations monitors in Syria, General Robert Mood, said he was worried about civilians trapped there.

Residents say Syria's third largest city has been surrounded by the army and pummeled by mortar and artillery fire almost daily since early June.

Tens of thousands of people have already fled to other cities to escape months of bombardment and street fighting.

"Contacts have been made with the leadership of the international monitors, in cooperation with the local Syrian authorities in the city of Homs to bring out these Syrian citizens," Syria's Foreign Ministry said.

"But the efforts of the monitors were unsuccessful... because the armed terrorist groups obstructed their efforts," it said, using a label Damascus employs to describe rebels who have joined the 15-month uprising and seek to topple Assad by force.

It accused the rebels of using civilians as "human shields".

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