State television confirmed a helicopter had crashed in Damascus but gave no details. Opposition activists said rebels had shot it down. Opposition video footage showed a crippled aircraft burning up and crashing into a built-up area, sending up a pillar of oily black smoke.
A day after his enemies accused Assad's troops and sectarian militia of massacring hundreds of people in the town of Daraya near Damascus, the possible shooting down of the helicopter, the latest of several such successes claimed by lightly armed rebel fighters, bolstered morale. But, witnesses said, even more intense army bombardments followed.
"It was flying over the eastern part of the city and firing all morning," an activist calling himself Abu Bakr told Reuters from near where the helicopter came down in the eastern suburbs. "The rebels had been trying to hit it for about an hour," he said. "Finally they did."
Video footage carried the sound of people celebrating the helicopter's dive with shouts of "Allahu akbar" (God is great).
Although rebel commanders have asked foreign allies for anti-aircraft missiles, Western nations are unwilling to supply such weapons for fear of them falling into hostile hands. There was no indication fighters in Damascus had used any missiles.
Army helicopters had been rocketing and strafing crowded working class suburbs on the eastern outskirts of the city since Sunday. Generally seen as rebel strongholds, they came under renewed assault early on Monday.
"The sound of gunfire and mortar shells exploding hasn't stopped," an opposition activist, Samir al-Shami, said from the area. "I see smoke rising everywhere."
Another activist based in the eastern suburbs, Mohammed Doumany, said: "There are constant explosions and blasts from mortars. The rebels are attacking security force checkpoints in the suburbs."
At least 32 people were killed in the area on Monday, opposition activists said. Video from campaigners showed 20 bodies on the floor of a mosque, including three children.
On Sunday, opposition activists said they had found about 320 bodies, including some of women and children, in Daraya, just southwest of Damascus. Most had been killed execution-style, they said. Videos on the Internet showed rows of bodies wrapped in sheets. Most seemed to be young men, but at least one video showed several children who appeared to have been shot.
Due to restrictions on non-state media in Syria, it was impossible to verify the accounts independently.
The uprising, which began as peaceful protests, has become a civil war. United Nations investigators have accused both sides of war crimes but laid more blame on government troops and pro-government militia than on the rebels.
The killings in Daraya, a working class Sunni Muslim town that sustained three days of bombardment before being overrun by the army on Friday, pushed the daily death toll to 440 people on Saturday, one of the highest since the uprising began, an activist network called the Local Coordination Committees said.
The official state news agency said: "Our heroic armed forces cleansed Daraya from remnants of armed terrorist groups who committed crimes against the sons of the town."
The death toll for the following day, Sunday, was about 200, including civilians and fighters, according to another activist network, the Syrian Observatory for Human Rights.
Clashes are raging across Syria as the 17-month-old rebellion grows increasingly bloody, particularly in Aleppo, Syria's biggest city and its economic hub, where the army and rebels appear stuck in a war of attrition.
Fighting in the northern city on Sunday was the heaviest in the past week, according to Reuters journalists there. Helicopters were circling and firing occasionally on Monday.
Reuters journalists saw a fighter jet fire on an eastern neighborhood of the city for two hours. Activists said southern districts of Aleppo were also repeatedly attacked on Monday.
"The front line has not changed. We cannot progress due to a lack of ammunition," said Abu Walid, a rebel commander in Aleppo. "All we can do is hold our positions."
Rebels say they control at least half the city of 2.5 million, but their hold is fragile since Assad's forces can unleash their air power and artillery against fighters who are comparatively lightly armed.
Assad, who met an Iranian parliamentary delegation in the capital on Sunday, said the crisis was the result of Western and regional states trying to crush Syria's role in the "resistance" against Western and Israeli domination in the region.
The United Nations says more than 18,000 people have been killed in the conflict that pits a mainly Sunni opposition against a ruling system dominated by the Assad family and other members of the Alawite faith, an offshoot of Shi'ite Islam.
Diplomatic efforts to stop the violence in Syria are stalled by a stalemate between Western countries, Sunni-led Gulf Arab states and Turkey - which all support the opposition - and Shi'ite Iran, which backs Assad, as do Russia and China.
With veto-wielding Russia leading resistance to action against Assad, the U.N. Security Council remains deadlocked.
Egypt is seeking to arrange a four-way meeting with Turkey, Saudi Arabia and Iran, the main regional heavyweights. Iran, a Shi'ite power, is Assad's main backer, while Saudi Arabia is believed to be supplying weapons to the rebels.
Syria's minister for national reconciliation was quoted by an Iranian news agency as praising Tehran's support and saying: "We face armed parties acting within the framework of an American-Israeli conspiracy, and we will destroy them, because they are trying to degrade Syria by causing internal and tribal wars.
He urged Egypt, now led by a Sunni Islamist president after decades of Western-backed military rule, to distance itself from those states hostile to Damascus if it wanted to broker a peace.