BEIRUT, Lebanon — Syrian insurgents attacked military checkpoints and other targets in parts of central Damascus on Wednesday, antigovernment activist groups reported, shattering a lull in the fighting as prospects for any talks between the antagonists appeared to dim.

The outbreak came a week after the opposition coalition’s top political leader first proposed the surprise idea of a dialogue with President Bashar al-Assad’s government aimed at ending the civil war. Frustrated by the government’s failure to respond definitively, the opposition leader, Sheik Ahmad Moaz al-Khatib, gave it a Sunday deadline.

Some antigovernment activists described the resumption of fighting, which had lapsed for the past few weeks, as part of a renewed effort by rebels to seize control of central Damascus, the Syrian capital, although that depiction seemed highly exaggerated. Witness accounts said many people were going about their business, while others noted that previous rebel claims of territorial gains in Damascus had almost always turned out to be embellished or unfounded.

Representatives of the Military Council of Damascus, an insurgent group, said that at least 33 members of President Assad’s security forces in Damascus had surrendered, while others had fled central Al Abasiyeen Square, and that other government forces had erected roadblocks on all access streets to the area to thwart the movement of rebel fighters.