At least a dozen Syrian secret police have defected from an intelligence compound, activists said, in what appeared to be the first major desertion from a service that has acted as a pillar of President Bashar al-Assad's rule.
A gunfight broke out overnight on Saturday after the defectors fled the Airforce Intelligence complex in the centre of Idlib city, 280 km (175 miles) northwest of Damascus.
Ten people on both sides were killed or wounded, the activists said on Sunday.
The defections came as the Arab League once again chided Syria for failing to sign up to a league-backed plan to end the violence in Syrian cities.
"We are very clear after the meeting yesterday... We give the Syrians one day, and I hope we will receive the answer from them. But until now I think there has been no answer from Syria," the diplomat said.
The Arab League had told Syrian authorities to sign an initiative to end the military crackdown on popular protests by Sunday, threatening to impose financial and economic sanctions if it does not sign soon.
A senior Arab diplomat at the League said late on Sunday that there was no sign Syria had responded to the deadline.
Such deadlines have slipped repeatedly in the past. Damascus complains that its sovereignty would be compromised by the plan, which would require it to admit Arab monitors to ensure that Syria pulled troops out of cities.
"There are letters still being exchanged between the Arab League and Damascus to reach a vision for the protocol... These communications and correspondence are being studied by Damascus," Foreign Ministry spokesman Jihad al-Makdesi said in the Syrian capital.
Assad has so far shown no sign of halting the crackdown on protests against his rule.
In Homs's Sunni district of Bab Amro on Sunday, several thousand people encircled the coffin of Khaled al-Sheikh, a 19-year-old protester who residents said was killed in random shooting by the army on the neighbourhood this week.
Abdelbassel Sarout, a 21-year-old soccer player, kissed Sheikh's bloody head as the mostly young crowd of men and women chanted to the beat of drums: "Sleep easy we will continue the struggle... mothers weep for Syria's youth."
"When we film the protests to send on YouTube, most demonstrators would try to hide their face so they would not be identified by the security police," Wael, a young activist, said. "Khaled was always barefaced, chanting the loudest."
"Not a single opposition neighbourhood was spared today. Troops either entered districts and raided houses, fired from roadblocks or tanks and pickup trucks hit houses and shops with machineguns," said Abu Zeinab, an activist in the city.
Syrian authorities say they are fighting foreign-backed "terrorist groups" trying to spark civil war who have killed some 1,100 soldiers and police since March.
The official state news agency said a father and three children, who local activists said were shot dead by militiamen loyal to Assad in a drive-by shooting, were killed by a "an armed terrorist gang" that broke into their house.
"We see from this heinous crime that the terrorists are continuing to commit their crimes with cold blood," the agency said.
Opposition sources said another 16 soldiers defected from Idlib on Sunday and fighting separately broke out between a new group of defectors, of similar size, and loyalist forces to the south, in the Josieh area on the border with Lebanon
They estimate the number of defectors from the military so far at several thousand, mainly army recruits from Syria's Sunni Muslim majority. Members of Assad's minority Alawite sect, an offshoot of Shi'ite Islam, have a tight grip on the country's military and security apparatus.
The sectarian dimension to the unrest has come to the fore after tit-for-tat sectarian killings were reported near Homs, a nascent insurgency broke out in the provinces of Homs, Deraa and Idlib, and the United Nations warned of the risk of a civil war.
U.S. Assistant Secretary of State Jeffrey Feltman accused Assad of forcing his Alawite sect into a bloody conflict with the country's majority Sunnis.
"Bashar al-Assad ... seems to be intent on fulfilling his own prophecy that Syria is going to move into chaos and civil war," Feltman, who is in charge of near eastern affairs, told reporters in the Jordanian capital Amman.
"(Assad) is using one community in Syria, he is drawing on intelligence services that are largely made up of one community in Syria and he is using them against demonstrations that are largely composed of another community in Syria," Feltman said.
The top U.N. human rights forum has condemned Syria for "gross and systematic" violations by its forces, including executions and the imprisonment of some 14,000 people.
Protests, modeled on "Arab Spring" revolts that have toppled leaders in Tunisia, Egypt and Libya, have continued in Homs and scores of cities and towns. Armed resistance has grown alongside the sustained peaceful demonstrations.
"The street still wants the protests to continue to maintain the moral edge of the uprising. But it does not mind if the revolt acquires armed teeth to protect the demonstrators and deter attacks by the army and security police," activist Talal al-Ashqar said by phone from Damascus.
Assad repeatedly has said he is battling to preserve Syria's sovereignty against a foreign conspiracy to sow sectarian strife. His isolation has deepened, with the Arab League, the European Union, the United States and Turkey piling on tougher and tougher economic sanctions.
But the 46-year-old president does not face any immediate threat of foreign military strikes. The West has shown no appetite for the type of intervention that helped oust Libya's Muammar Gaddafi.