Arab League peace monitors headed to three more Syrian cities Thursday to check whether government forces have halted violence against protesters as Damascus promised after observers in the centre of the unrest were mobbed by people demanding protection.

The cities of Deraa, Hama and Idlib lie along a 450 km (280 mile) arc of revolt from the south to the north of Syria. At its middle point is Homs, where the Arab mission's second day had a controversial start when its Sudanese leader said Wednesday he saw nothing frightening in his initial tour of the city.

Opposition activists say Homs has seen the worst of the violence since the revolt began nine months ago, with army tanks in action and three dozen people killed the day before the monitors arrived.

The Arab mission is the first significant international involvement in Syria's conflict, in which thousands have been killed in a military crackdown on the uprising against 41 years of rule by the family of President Bashar al-Assad.

People really hope to get to reach them. We do not have much access to the team. The people stopped believing anything or anyone now. Only God can help us now, said Abu Hisham, an opposition activist in Hama.

He said people were going down into the streets in Hama to await the delegation. Security was heavy and marksmen were seen on rooftops.

Hama has a haunting resonance to Syrians opposed to Assad because up to 30,000 people were massacred there in 1982 when his father, president at the time, stormed the city with armoured forces to suppress an Islamist uprising.

A source in the Arab League mission's operations centre in Cairo said there had been a problem with communications but the monitors' schedule was holding up.

We have contacted our teams ...Today's plan will not be changed and the only problem we faced today was the bad phone network, which made our communication with the monitors harder. It took more time to reach them and determine their locations, the source told Reuters.

ROCKY START

The mission Wednesday got off to a rocky start when observers were rejected by residents of one of the worst-hit quarters of Homs, Syria's third largest city with one million people, because they showed up with an army escort.

Then they were mobbed in their car, and at another moment even appeared to come under fire.

They finally went back to Baba Amr district to see its shattered houses and hear from people who have lost loved ones. One family showed them a dead boy.

In Deraa, Hama and Idlib, all of which have seen killings of late, the observers are likely to hear similar accounts.

But their chief, Sudanese general Mustafa al-Dabi, has raised international concern for the mission's credibility with a comment that he had seen nothing frightening on his first, very brief initial trip to Homs Tuesday.

This was only the first day and it will need investigation. We have 20 people who will be there for a long time, he said.

After at first expressing concern over his remarks, the United States said the mission must be given time.

It was just Day One, State Department spokesman Mark Toner said. We need to let this mission get up and running, let them do their job and then let them give their judgement.

Most of the 5,000 or more people estimated by the United Nations to have been killed in Syria since March lost their lives in four hard-hit districts of Homs to machinegun fire, sniper bullets, mortar blasts and tank shelling, or torture.

Unless it can establish its credibility by proving it has unobstructed access to all areas and is able to hear uncensored accounts, the Arab League mission may not be able to satisfy all sides that it can make an objective assessment of the crisis.

INSURGENCY RISING

Relentless military attacks on peaceful civilian protests have bred armed insurgency in some as thousands of army defectors in the recently-raised Syrian Free Army (FSA) attack military and police convoys, bases and checkpoints.

A gritty video shot by rebels in Deraa showed the ambush of a security forces convoy Wednesday by eight gunmen who opened fire from a rootfop. The Syrian Observatory for Human Rights said four soldiers were killed in the attack by the FSA on a road near the southern village of Dael in Deraa province.

Assad says he is combating Islamist militants steered from abroad. He says over 2,000 security force men have been killed.

Syria resisted outside involvement for months but yielded to unprecedented pressure from fellow members of the 22-state Arab League last month, agreeing to let the monitors in to witness withdrawals of forces from the turbulent cities.

But the killing did not stop, before and during the first two days of the monitoring mission, and opposition activists predict it will not stop after the observers are gone in a month. There will be fewer than 200 observers to witness and interview victims of violence scattered along the populated areas of a sprawling country of 23 million.

The Arab League is under pressure to prove it can make a significant difference. Its secretary general, Nabil Elaraby, has said the mission is in uncharted territory, but in a week's time, from the start of the operation, we will know if Syria is in compliance with its peace pledge. We don't need a month.

If the mission cannot credibly certify to the world that Assad is reining in his forces with a genuine will to negotiate reform with his opponents, the U.S. State Department has said other means of international action will be pursued.

The West has demanded Assad step down, but Russia and China oppose interference. Assad also enjoys an alliance with Iran.

Increasingly isolated, Assad has lost the trust of his big neighbour Turkey, which has called for him to quit and spoken of creating a buffer zone, and from whose territory rebel forces now launch attacks.

(Additional reporting by Ayman Samir in Cairo; Writing by Douglas Hamilton; Editing by Mark Heinrich)