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 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.

OBAMA WARNS ASSAD

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.

“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. 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 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.

The discovery of bodies in similar condition is not uncommon in Syria, particularly in the last few months as the uprising descended into a civil war with heavy sectarian undertones.

TANK AND MORTAR SHELLING

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 along with a tank defected to the rebels’ side in Moadamiyeh on Sunday, which may have been the reason for Monday’s shelling.

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.

The rights groups and activists said the latest assaults by tanks and warplanes caused two houses to collapse in Aleppo, Syria’s largest city, killing at least 14 people. The buildings were in the Al-Sakhour and Qadi Askar neighborhoods, said activist Mohammed Saeed, reached by Skype inside the city. .

Aleppo has been the scene of daily battles for several weeks now, with forces loyal to Assad trying to wrest control from the rebels without making much headway.

In the southern city of Daraa, birthplace of the uprising, intense fighting between government troops and rebels killed 16 people, including two children and two women, the activist groups said.

AIRSTRIKES INCREASING

As it battles for survival against the rebels, the regime has increasingly resorted to the use of airstrikes, particularly in the north around Aleppo where rebels have seized territory.

Fighter jets Monday bombed the town of Tel Rifat, 20 miles north of Aleppo. The town serves as the headquarters of one of the largest rebel groupings.

The bombing punched a crater in the courtyard of a high school and vocational school for girls and leveled five nearby homes. An adjacent elementary and middle school for girls was also damaged, apparently by strafing from fighter jets.

No one was killed or injured in the airstrike, because the school and the homes were empty. Most of the town’s 35,000 people have fled due to frequent airstrikes and shelling.

“They don’t want there to be an educated society,” said Hatem, who taught Islamic education at the school and came to inspect the damage. “They want people to be ignorant so they don’t ask for their rights.”