Bombs killed at least 14 prisoners in a Syrian security vehicle Saturday, and fierce battles erupted between rebels and state forces as the Arab League considered whether to keep monitors in place.
Rebels seized parts of a suburb of Damascus late on Saturday, activists said, and fighting continued into the night.
The League looks set to extend its monitoring mission in Syria, given the lack of any Arab or world consensus on how to halt the bloodshed there, an Arab diplomatic source said.
The Syrian Observatory for Human Rights, which tracks the 10-month-old revolt against President Bashar al-Assad, said an explosive device planted on a road in the northwestern province of Idlib had killed 15 detainees and wounded dozens.
Syria's state news agency SANA said a terrorist group had set off two explosions on the road between the towns of Idlib and Ariha, killing 14 prisoners and wounding 26. Six police guards were also wounded, some critically.
Activists in Idlib offered a very different account, saying the vehicle had actually been carrying dead bodies. They uploaded videos of corpses on the bloodied floors of a hospital morgue, some of which appeared to be decomposing, and said they had come from the vehicle.
Foreign journalists are mostly banned from Syria and such reports are impossible to verify.
Elsewhere in Idlib, clashes broke out between rebels and troops in the city of Maarat Noaman.
Ten soldiers were trying to desert and their escape sparked clashes between the army and the rebels. One rebel was martyred when he helped give the defectors cover and nine army personnel were killed, the Observatory's head Rami Abdelrahman told Reuters by telephone from Britain.
The Observatory said troops had clashed with army deserters who had joined the insurgency in the town of Jebel al-Zawiya, also in Idlib province, which borders Turkey.
The group said late Saturday insurgents were fighting the army at the entrances to the Damascus suburb of Douma, which has been a centre of protest.
No one can get in or out of Douma right now. This is the first time the rebels do anything more than hit-and-run attacks. Tonight they started making barriers in the streets and every few minutes I hear gunfire and explosions, one said.
Syria accuses its neighbours of failing to combat arms smuggling to insurgents across their borders. Saturday Syrian forces killed a Lebanese fisherman and wounded another when they seized their boat at sea, the father of the dead man said.
Residents said the Syrians may have suspected the men of smuggling.
For graphic on Arab League http://link.reuters.com/pev65s
Hundreds of people have been killed during the month-long observer mission, despatched to assess Syria's implementation of an Arab peace plan originally agreed in early November.
Sudanese General Mohammed al-Dabi, head of the 165-strong monitoring team, was due in Cairo Saturday to submit his report for a League committee on Syria to consider Sunday.
Syria is keen to avoid tougher action by the Arab League or the United Nations. It has tried to show it is complying with the plan, which demands a halt to killings, a military pullout from the streets, the release of detainees, access for the monitors and the media, and dialogue with opposition groups.
Critics say the Arab monitors have only given Assad diplomatic cover to pursue a bloody crackdown on his opponents.
The opposition Syrian National Council (SNC) told Reuters it had formally asked the League to refer the Syrian crisis to the U.N. Security Council.
But an Arab source said the League was most likely planning only to extend the mission's mandate: Yes, there is not complete satisfaction with Syria's cooperation with the monitoring mission. But in the absence of any international plan to deal with Syria, the best option is for the monitors to stay.
This month the Syrian authorities have freed hundreds of detainees, announced an amnesty, struck a cease-fire deal with armed rebels in one town, allowed the Arab observers into some trouble spots and admitted a gaggle of foreign journalists.
Assad also promised political reforms, while vowing iron-fisted treatment of the terrorists trying to topple him.
Burhan Ghalioun, head of the SNC, was in the Egyptian capital for meetings with opposition colleagues and Arab League officials.
The group said in a statement he would ask for the case to go to the Security Council in order to get a resolution imposing a no-fly zone or safe zone.
Western powers have failed to overcome Chinese and Russian opposition to any Security Council resolution condemning Syria or imposing sanctions.
The United States and the European Union have toughened their own punitive measures, but have shown no desire to mount a Libya-style military intervention to help Assad's opponents, who include both armed insurgents and peaceful protesters.
Washington warned Friday that it might soon close its embassy in Syria due to worsening security conditions and said it believed Assad no longer had full control of the country.
U.S. concern about the safety of its mission in Damascus, which was attacked by a pro-Assad crowd in July, intensified after three deadly blasts in the Syrian capital in recent weeks, blamed by Syrian authorities on al-Qaeda suicide bombers.
Closing the embassy would not amount to cutting diplomatic ties, but would reduce direct U.S. contacts with Damascus.
A White House spokesman said Assad's fall was inevitable and demanded he halt violence against protesters in which the United Nations says more than 5,000 people have died since March. Syria says 2,000 security personnel have been killed. (Writing by Alistair Lyon and Erika Solomon; editing by Andrew Roche)
(Editing by Andrew Roche)