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 over 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.
Salam Mohammed, an activist in Damascus, described Al Abasiyeen Square as "on fire," and a video clip uploaded on YouTube showed a thick column of black smoke spiraling over the area while the sound of shelling could be heard. A voice is heard saying the shelling had started a fire. The Local Coordination Committees, an anti-Assad activist network in Syria, also reported gunfire in nearby streets.
Firas al-Horani, a military council spokesman, said fighters of the Free Syrian Army, the main armed opposition group, were in control of Al Abasiyeen Square. He also said, "The capital, Damascus, is in a state of paralysis at the moment, and clashes are in full force in the streets."
It was impossible to confirm Mr. Horani's assertions or the extent of the fighting because of Syrian government restrictions on foreign news organizations. But Syria's state-run media said insurgent claims of combat success in Damascus were false. "Those are miserable attempts to raise the morale of terrorists who are fleeing our valiant armed forces," said SANA, the official news agency.
Deadly violence also was reported in the Homs Province town of Palmyra, the site of a notorious prison where Mr. Assad's father, Hafez, ordered the summary execution of about 1,000 prisoners during an uprising against his family's grip on power in the 1980s.
The Syrian Observatory for Human Rights, a Britain-based group with a network of contacts inside Syria, said two booby-trapped cars exploded near the military intelligence and state security branches, killing at least 12 members of the security forces and wounding more than 20. The observatory said government forces deployed throughout Palmyra afterward, engaging in gun battles with insurgents that left at least eight civilians wounded in the cross-fire.
SANA also reported an attack but said it was caused by two suicide bombers who had targeted a residential part of the town, killing an unspecified number of civilians.
The new mayhem came as discord appeared to grow within the National Coalition of Syrian Revolutionary and Opposition Forces, the umbrella anti-Assad group, over a proposal made on Jan. 30 by Sheik Khatib, its leader, to engage in talks with Mr. Assad's government aimed at ending the nearly two-year-old conflict, which has left more than 60,000 people dead.
Although Sheik Khatib's proposal contained a number of conditions, including the release of prisoners, it broke a longstanding principle that Mr. Assad must relinquish power before any talks can begin.
Many of Sheik Khatib's colleagues grudgingly agreed to go along with the proposal after it had been made, but critical voices have been rising, especially among the coalition's more militant elements, who have never trusted Mr. Assad and have concluded that any negotiation is a waste of time.
In what appeared to be an acknowledgment of the discord, Sheik Khatib said in an interview with the BBC's Arabic service from his headquarters in Cairo on Wednesday that his own patience with the Syrian government was running out. Sheik Khatib gave the government until Sunday to release prisoners, especially women, or "the initiative will be broken."
In a new video uploaded on YouTube, a cleric from the Nusra Front, an anti-Assad Islamist militant group that the Obama administration has classified as a terrorist organization, said in a prayer speech that brute force against Mr. Assad and his disciples was the only solution.
"We will cut their heads, we swear to kill them all, and they will see our worst war," said the cleric, who spoke in Libyan-accented Arabic at a mosque in the contested northern city of Aleppo, holding a sword in his right hand. "No for the negotiations, no for the talks, no retreat in a jihad, for God's sake."
Hania Mourtada reported from Beirut, and Rick Gladstone from New York. Reporting was contributed by Hwaida Saad from Beirut; Hala Droubi from Dubai, United Arab Emirates; and Karam Shoumali from Antakya, Turkey.
This article originally appeared in The New York Times.