Syrian government forces heavily shelled the cities of Aleppo and Daraa and a suburb of Damascus on the second day of a major Muslim holiday Monday, killing up to 30 people, rights groups and activists said.

There was a relative lull in the civil war on Sunday, the first of three days of the Eid al-Fitr holiday marking the end of the holy month of Ramadan. During the holiday, Muslims the world over celebrate by wearing new clothes, feasting on sumptuous food and visiting the graves of loved ones. The renewed fighting, however, showed President Bashar Assad's regime is not letting up on its drive to quell the 18-month-old uprising out of respect for the occasion.

Activists reported no signs of jubilation across the battered nation, with smaller-than-usual turnout for traditional prayers on the first day of the holiday and an air of gloom blanketing major cities.

Adding to the despair, two main activist groups — The Syrian Observatory for Human Rights and the Local Coordination Committees — said that 10 bodies of adult males shot execution style were found in the Qaboun district in the capital Damascus. 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.

Anti-regime activists say some 20,000 people have been killed since the revolt against Assad's rule began in March 2011.

New Syrian envoy

Even the UN's new envoy to Syria acknowledged on Sunday that he had no concrete ideas to end the conflict and that his mission would be difficult without a unified position by the UNSecurity Council.

"The problem is not what I can do differently, it is how others are going to behave differently," Lakhdar Brahimi told The Associated Press at his Paris home on Sunday. "If they spoke in one voice and were clearly supportive of what I will be doing on their behalf, that is what I need," Brahimi said of what he seeks from the Security Council. "Without a unified voice from the Security Council, I think it will be difficult," the former Algerian foreign minister added.

Brahimi was named Friday to replace former UN Secretary-General Kofi Annan as peace envoy to Syria. He served as a UN envoy in Afghanistan and Iraq and helped negotiate the end of Lebanon's civil war as an Arab League envoy. He said Annan's mission failed "because the international community was not as supportive as he needed them to be."

Russia and China have used their veto power at the Security Council to block strong Western- and Arab-backed action against the regime of Syria's Assad.

A Syrian foreign ministry source quoted by the official SANA news agency warned Brahimi that, for his mission to succeed, he must persuade countries backing the rebels to stop their support for the "armed terrorist bands" — the regime's parlance for the rebels.

Syria often singles out Saudi Arabia, Qatar and Turkey as the rebels' main backers.

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 neighbourhoods, said activist Mohammed Saeed, reached by Skype inside the city.

Obama says U.S. policy could change

U.S. President Barack Obama says thinking on possible military involvement in Syria would change if chemical or biological weapons came into play in the Syrian civil war. Speaking to reporters on Monday, Obama said the use of such weapons of mass destruction, which Syria has, 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," he said.

Obama warned Assad’s regime and "other players on the ground" that the use or movement of such weapons would be a "red line" for the United States.

Daily battles

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.

Saeed also said that fighting raged inside the city with rebel forces making advances in the districts of Al-Jadidah and Maadi Telal, in the northeastern part of Aleppo.

The reports from the activists and groups — the British-based Syrian Observatory for Human Rights and the Local Coordination Committees — could not be independently verified.

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

An activist in the Damascus area, El-Said Mohammed, said seven people were killed and at least 70 wounded when government forces shelled the suburb of Moadamiyeh with tanks and mortars. He said some 30 troops along with a tank defected to the rebels' side on Sunday, which may have been the reason for Monday's shelling.

Mohammed spoke by Skype from the greater Damascus area. His information could not be verified, but the Observatory said the shelling in Moadamiyeh killed at least 10 civilians and three rebels.