Britain called on Wednesday for harsher sanctions on Syria, where an Arab monitoring mission has failed to halt bloodshed in a 10-month-old revolt against President Bashar al-Assad.

Syria may let the monitors stay on after their mandate expires on Thursday, but Assad's foes say the Arab League peace effort has failed and the U.N. Security Council should step in.

Arab foreign ministers, due to consider their next step at the weekend, are split over how to handle Syria, as is the Security Council, which has failed to adopt any position.

U.S. President Barack Obama has again called for a change of government, saying the violence in Syria was unacceptable.

Prime Minister David Cameron accused Iran and Lebanon's Shi'ite Hezbollah movement of helping to prop up Assad, whom he described as a wretched tyrant.

Britain needs to lead the way in making sure we tighten the sanctions, the travel bans, the asset freezes, on Syria, Cameron told parliament in London.

European Union foreign ministers are expected to discuss extra EU sanctions at a meeting on Monday.

Hundreds of killings on both sides have been reported since the Arab League sent observers last month to see whether Damascus was respecting a peace plan it accepted on November 2.

An Arab League source said Damascus would accept a one-month extension of the monitoring mission, but no broadening of its mandate. Critics say the observers have only provided Assad with diplomatic cover and more time to crush his opponents.

The opposition Syrian Observatory for Human Rights said security forces killed a civilian in a village in the northwestern province of Idlib on Wednesday and the body of a youth detained nearly two weeks ago turned up in Homs.

It said a soldier had been killed and five wounded in clashes between troops and army deserters in the Idlib village of Khaf Takharim. Three rebel soldiers were also wounded.

Syria's state news agency SANA said the strangled body of a veterinarian doctor was found in Homs bearing marks of torture four days after he was kidnapped by an armed terrorist group.

The United Nations said on December 13 that Assad's security forces had killed more than 5,000 people since the unrest erupted in mid-March. Nine days later, the government said armed terrorist groups had killed 2,000 security personnel.

FEARS OF CIVIL WAR

Lebanese Druze leader Walid Jumblatt, who has distanced himself from Damascus in recent months, said the Arab monitors had failed to staunch the bloodletting and that Syrians wanted freedom, like other Arabs who have revolted in the past year.

I am more and more concerned about the possibility that Syria will plunge into more violence and ... maybe civil war, he told Reuters in an interview.

The Arab plan required Syria to halt the bloodshed, withdraw troops from cities, free detainees, provide access for the monitors and the media and open talks with opposition forces.

A tenuous truce was holding on Wednesday in Zabadani, near the Lebanese border, where troops had been fighting anti-Assad rebels, residents said. But heavy machinegun fire and explosions rocked the troubled city of Homs, an opposition group said.

As of now there is no shelling and no gunfire. It is quiet. But the army is still surrounding the area, said one Zabadani resident who gave her name as Rita.

Syrian forces backed by tanks attacked the hill resort on Friday in the biggest military offensive against insurgents since the Arab monitors began work on December 26.

Michel Kilo, a dissident Syrian writer who spent six years in jail, said the struggle in Syria was at an impasse.

The regime can't stop people protesting and the people can't bring the regime down, he told France's Le Figaro daily, adding that Assad wanted to bring in Iran, Hezbollah, Iraq and to threaten Gulf countries with a long war.

Riad al-Asaad, a leader of the rebel Free Syrian Army, told Reuters on Tuesday the Arab League's efforts had failed.

We call on them to turn the issue over to the U.N. Security Council and we ask that the international community intervene because they are more capable of protecting Syrians at this stage than our Arab brothers, the former army colonel said.

The Arab League source said China and Russia, which have blocked Security Council action so far, had urged President Assad to accept an extension of the monitoring mission to avert an escalation at the international level.

Qatar has proposed sending in Arab troops, an idea rejected by Syria and one likely to be resisted by its Arab allies.

(Additional reporting by Ayman Samir in Cairo, Khaled Yacoub Oweis in Amman, Mariam Karouny and Dominic Evans in Beirut and Keith Weir in London)