BEIRUT – In a barrage of mortar shells, Syrian forces killed 200 people and wounded hundreds in Homs in an offensive that appears to be the bloodiest episode in the nearly 11-month-old uprising, activists said today.

The assault in Homs, which has been one of the main flashpoints of opposition during the uprising, comes as the U.N. Security Council prepares to vote on a draft resolution backing an Arab call for President Bashar Assad to give up power.

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A protester holds a poster depicting Russian Prime Minister Vladimir Putin and Iranian supreme leader Ayatollah Ali Khamenei pumping up Syrian President Bashar Assad. The Associated Press

Two main opposition groups, the British-based Syrian Observatory for Human Rights and the Local Coordination Committees, said the death toll in Homs was more than 200 people in shelling that began late Friday.

“This is the worst attack of the uprising, since the uprising began in March until now,” said Rami Abdul-Rahman, the head of the Observatory, which tracks violence through contacts on the ground. The reports could not be independently confirmed.

It was not clear what precipitated the attack, but there were reports that army defectors set up checkpoints and were trying to consolidate control.

Earlier on Friday, deadly clashes erupted between government troops and rebels in suburbs of the Syrian capital and villages in the south, sparking fighting that killed at least 23 people, including nine soldiers, activists said.

Assad is trying to crush the revolt with a sweeping crackdown that has so far claimed thousands of lives, but neither the government nor the protesters are backing down and clashes between the military and an increasingly bold and armed opposition has meant many parts of the country have seen relentless violence.

The U.N. Security Council will meet this morning to take up a much-negotiated resolution on Syria, said a diplomat for a Western.

The move toward a vote came after Secretary of State Hillary Rodham Clinton spoke by telephone with Russian Foreign Minister Sergey Lavrov in an effort to overcome Russian opposition to any statement that explicitly calls for regime change or a military intervention in Syria.

The United States and its partners have ruled out military action but want the global body to endorse an Arab League plan that calls on Assad to hand power over to Syria’s vice president.