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 which marks the end of the holy month of Ramadan. The renewed fighting showed President Bashar Assad's regime is not letting up on its drive to quell the 17-month-old uprising out of respect for the occasion.

In Washington, President Barack 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.