TEL RIFAT, Syria — Government forces pummeled the battered city of Aleppo with airstrikes and tanks and shelled parts of Damascus and southern Syria on 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 Obama said US thinking on military involvement in Syria would change if chemical or biological weapons came into play in the civil war. He told reporters that the use of such weapons of mass destruction would widen the conflict considerably.

‘‘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’’ against the use or movement of such weapons.

‘‘We have put together a range of contingency plans,’’ Obama said. “We have communicated in no uncertain terms with every player in the region that that’s a red line for us.’’ The United States has been reluctant to intervene militarily so far.

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 was seen as a sign of desperation as Assad’s grip on power slipped. It came shortly after rebels assassinated four of the president’s top security officials, the biggest blow to the regime in the entire uprising.

Since the holiday began on Sunday, an air of gloom has blanketed the nation, and activists said there have been no signs of jubilation.

Adding to the despair, 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, which included two children, were found on the side of a road with clear signs of torture on their bodies. Some were naked, others handcuffed.

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 that as many as 31 people were killed.