Arab League foreign ministers have called on Syrian President Bashar al-Assad to step down, as rebels say they are encouraged by last week's assassinations in Damascus.
The rebels undercover near the Syrian capital said that the deaths of four top officials were a severe blow to the government.
The BBC correspondent, Paul Wood reported that Islamist rebels were receiving weapons and money from outside.
Fighting continued overnight.
Government forces recaptured parts of Damascus, the suburbs of Barzeh and Mezzeh, which had fallen into rebel hands.
There and in other quarters, activists said a number of suspected rebels or sympathisers were summarily executed.
Fighting was also reported in Syria's second city, Aleppo.
After an emergency meeting in Qatar, Arab League foreign ministers called on President Assad to resign rapidly, and offered his family safe passage out of Syria.
They also called on the Syrian opposition to form a transitional government.
However, the BBC's Jim Muir, in neighbouring Lebanon, said that the call appeared to have fallen on deaf ears.
Mr Assad held a meeting with his new army chief of staff and gave him instructions, reportedly including a drive to crush armed rebels.