TEL RIFAT, Syria - Government forces pummeled the battered city of Aleppo with airstrikes and tanks and shelled parts of Damascus and southern Syria Monday, killing at least 100 people during a major Muslim holiday, rights groups and activists said.
The violence escalated dramatically after a one-day lull on Sunday, the start of the three-day Eid al-Fitr holiday that marks the end of the holy month of Ramadan.
President Bashar Assad's regime did not let up on its drive to quell the 17-month-old uprising out of respect for the occasion.
In Washington, President Obama said U.S. thinking on military involvement in Syria would change if chemical or biological weapons came into play in the civil war.
He told reporters the use of such weapons of mass destruction would widen the conflict considerably.
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"It doesn't just include Syria. It would concern allies in the region, including Israel, and it would concern us," Obama said, warning the Assad regime and "other players on the ground" that the use or movement of such weapons would be a "red line" for the United States.
Last month, the Syrian regime confirmed for the first time that it possessed chemical weapons by threatening to use them in case of any foreign aggression.
The warning came shortly after rebels assassinated four of the president's top security officials, the biggest blow to the regime in the entire uprising.
Two main activist groups - The Syrian Observatory for Human Rights and the Local Coordination Committees - said that 12 bodies shot execution style were found in the Qaboun district in the capital Damascus. Activist Omar al-Khani said the bodies included those of two children.
Most of the deaths Monday were a result of tank and mortar shelling as well as clashes in the Damascus suburbs of Daraya and Moadamiyeh, where some activists reported the regime used helicopter gunships.
The Observatory and others said up to 31 people were killed.
An activist, El-Said Mohammed, said some 30 troops with a tank defected to the rebels in Moadamiyeh on Sunday, which may have spurred Monday's shelling.
Mohammed spoke by Skype from the Damascus area. His information could not be verified.
Both the Observatory, which relies on a network of activists on the ground, and the LCC reported at least 100 civilian deaths across the country, a heavy toll for a single day.
Anti-regime activists say some 20,000 people have been killed since the revolt against Assad's rule began in March 2011.
