Violence sweeping across Syria killed 25 people on Saturday, most of them in a battle between troops and a growing force of army defectors who have joined the movement to oust the autocratic ruler, President Bashar Assad, activists said.

The revolt against Assad's rule began with peaceful protests in mid-March, triggering a brutal crackdown. The unrest has steadily become bloodier as defectors and some civilians take up arms, prompting the United Nations' human rights chief to refer to it this week as a civil war and urge the international community to protect Syrian civilians.

Economic and diplomatic sanctions by the United States, the European Union, Turkey and the 22-member Arab League have so far failed to blunt the turmoil.

Arab officials met in the Gulf nation of Qatar to review punitive steps by the Arab League, agreeing to a set of sanctions against Syria late Saturday including setting a list of Syrian officials who will be barred entry from fellow Arab nations in the 22-member bloc. Syrian President Bashar Assad is not on the list.

Many of the sanctions announced last Sunday by Syria's Arab neighbours went into effect immediately, including cutting off transactions with the Syrian central bank, halting Arab government funding for projects in Syria and freezing government assets.

4,000 people killed so far

The worst violence on Saturday took place in the restive northwestern city of Idlib.

The pre-dawn clashes between regime forces and defectors killed seven soldiers and policemen, as well as five defectors and three civilians, according to a British-based group of Syrian activists called the Syrian Observatory for Human Rights.

Shell to leave Syria

Oil company Royal Dutch Shell said Friday it would halt all activities in Syria following European Union-imposed sanctions on the country's vital petroleum industry.

Although Syria produces less than one per cent of global oil output, it is a big earner for the regime, CBC freelance contributor Dominic Valitis reported from London.

"This latest news will certainly have an impact and will come as a blow to President Bashar Assad's regime," he said Friday, adding it will likely take the company a few weeks to shut down production in the country.

Elsewhere, security forces killed one civilian in the southern province of Daraa, six in the central region of Homs and three others in areas near Idlib, the observatory said.

The UN's top human rights official said this week that Syria is in a state of civil war and that more than 4,000 people have been killed since March.

Until recently, most of the bloodshed in Syria was caused by security forces firing on mainly peaceful protesters, but there have been growing reports of army defectors and armed civilians fighting regime forces.

November was the deadliest month of the uprising, with at least 950 people killed in gunbattles, raids and other violence, according to activist groups.

'Terrorists' smuggling weapons arrested

In the west of the country, Syrian troops detained at least 27 people in the village of Talkalakh on the border with Lebanon and set fire to the homes of nine activists who were on the run, the observatory said.

Talkalakh is within walking distance from Lebanon, and at least two Lebanese civilians were struck by bullets on their side of the border on Friday. Witnesses said that they had heard hours of explosions and heavy machine-gun fire coming from the village.

The country's state-run SANA news agency confirmed the arrests in Talkalakh, saying that those detained were "terrorists" involved in smuggling weapons, drugs and bringing in fighters from Lebanon. The regime has consistently blamed armed gangs acting out a foreign conspiracy for Syria's unrest.

The opposition activists reject that and say they are pushing for Assad's ouster in hopes of breaking open the nation's closed political scene.

The reports of new violence could not be independently confirmed. The regime has sealed the country off from foreign journalists and prevented independent reporting.