Government forces have hit back at rebel-held areas in Syria's two biggest cities - Damascus and Aleppo.
Sustained assaults were launched on Sunday against the north-eastern Damascus suburb of Barzeh, and against Mezzeh, in the west of the city.
Some reports from activists said troops regained control of Mezzeh, killing about 20 people suspected of helping the rebels.
Fighting was also said to be continuing in the country's second city, Aleppo.
The government counterattack in Syria follows a week in which rebels made major advances, taking control of several parts of Damascus, seizing border crossings and claiming an attack that killed four top security officials, including the defence minister and President Bashar al-Assad's brother-in-law.
The offensive against Mezzeh involved more than 1,000 troops and allied gunmen, backed by armoured vehicles, tanks and bulldozers, according to residents and activists quoted by the Reuters news agency.
Government forces "executed" at least 20 men in the area, some activists told the agency by telephone.
The attack on Barzeh, in the north-east of Damascus, was carried out by the army's fourth division, commanded by the president's brother Maher, using tanks and armoured personnel carriers, the Syrian Observatory reported.
Earlier on Sunday, state media said that all "terrorists" - as the government calls the rebels - had been "cleansed" from Qabun, a district east of Barzeh. State television showed extensive destruction.
The BBC's Jim Muir in Beirut says government forces seem determined to drive the rebel Free Syrian Army completely out of Damascus and are setting about it quite systematically.
Reports from activists in Aleppo said there were clashes overnight from Saturday to Sunday between the Free Syrian Army and security forces.
They said a building in the Seif al-Dawla district collapsed under tank fire.
State TV played down the scale of the violence, saying troops were merely hunting down "terrorists".
The commander of FSA forces in Aleppo province, Col Abdul Jabbar Mohammed Aqidi, vowed to "liberate" the whole city, called on government troops to defect and vowed to protect members of the president's minority Alawite sect.
There were also reports of violence in the eastern city of Deir al-Zour on Sunday. Witnesses told Reuters that it was being attacked with artillery and rockets from helicopter gunships.
BBC sources in Syria also confirmed that rebels were now in control of the Bab al-Salam border crossing with Turkey. Turkey is not allowing non-Syrian nationals through so the border remains effectively closed.
Early on Monday, Qatar's Prime Minister, Hamad bin Khalifa al-Thani, said a meeting of Arab League foreign ministers in his country had offered President Bashar al-Assad safe passage out of Syria if he stepped down quickly.
He also said the gathering had urged the Free Syrian Army rebels and the opposition to form a transitional government.
And according to a report in the Wall Street Journal on Sunday, the United States has been trying to persuade Iraq to close its air space to flights between Syria and Iran in order to stop arms and oil shipments from reaching Syria. The West suspects Tehran of supplying arms to President Assad.
On Sunday, the Syrian Observatory for Human Rights reported that at least 19,106 people had been killed since March 2011. The UN said in May that at least 10,000 people had been killed.