Syrian security forces fired on anti-government protests and killed 11 people on Thursday, as Arab League peace monitors arrived in two more cities to check on compliance with a plan to end bloodshed.

Killings were reported at rallies in towns across Syria, including four in Hama shortly after a monitoring team arrived. Two more people were shot in Idlib, another monitors' destination, as they passed a security checkpoint.

The cities to be visited on Thursday - Deraa, Hama and Idlib - lie along a 450-km (280-mile) arc of revolt from the south to the north of Syria. At its midpoint is Homs, where the monitors' second day had a controversial start when its Sudanese chief reported seeing nothing frightening in an initial tour.

The Arab mission is the first notable 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.

But questions have arisen about the monitors' credibility as their movements appear to be restricted.

Syrian state television reported that monitors had arrived in Hama and Deraa.

Shortly before, anti-Assad activists said they had seen no sign of monitors on the streets by mid-afternoon and they were unable to contact them by telephone. Extra security forces were deployed around restive areas expecting monitors, they said.

Where are they? We worked very hard for this visit, we got witnesses and documented deaths and sites of shelling. People wanted to march but the monitors are missing. The security presence is really strong - it looks like they have been preparing as much as we have, said activist Odei in Idlib.

In Hama, activists said protesters went down into the streets in Hama to await the Arab League delegation, amid heavy security with snipers pointing guns out of top floor windows.

People really hope to reach (the monitors). 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.

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, had armed forces raze part of the city to the ground to crush an Islamist uprising.

Residents in the Harasta suburb of the capital Damascus said they had spotted cars with an Arab League logo. If the monitors were indeed there, it would be their first surprise visit.

A source in the Arab League mission's operations centre in Cairo said earlier on Thursday 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.

WHERE WERE THEY?

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.

The United Nations estimates that at least 5,000 people, mostly civilian protesters, have been killed in nine months of unrest that Assad has sought to stamp out with tanks and troops storming into restive cities and towns.

Fifteen people died on the monitors' first day and activists, fearing the Arab League team will be ineffective, say they expect the bloodshed to continue until observers leave.

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

Syria's upheaval continues a series of popular Arab uprisings that have toppled three dictators this year.

In Homs, Syria's third largest city and epicentre of anti-Assad ferment, protesters said they were already fed up with the monitors who they said seemed unsympathetic and were hard to find even when touring their neighbourhoods.

This mission is a big lie. They say they were in Khalidiya neighbourhood. I haven't seen them, said Tamir, shouting by telephone over protester chants of down with the regime.

We've been here at the protest. Where were they?

The head of the main opposition group in exile, The Syrian National Council, met with Arab League officials in Cairo to discuss the monitoring mission.

The delegation should have been bigger because the points of confrontation and violence are much greater than the number of the monitors, and they should have better logistical means to enable them to move quicker, Burhan Ghalioun told reporters.

Some 150 monitors in all are expected to enter Syria by the end of the week. But activists say Syrian government or security officials escorting monitors can intimidate residents in protest hotbed areas and that there are not enough monitors to see the full scope of unrest in a large country of 23 million people.

Burhan Ghalioun blamed Assad's government for the monitors' restrictive conditions: Until now the Syrian regime has not changed its style of lies and tricks, he said.

The mission chief himself, Sudanese General Mustafa al-Dabi, raised international concern for the team's credibility when he said he had seen nothing frightening on a very brief initial trip to Homs on 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.

BUDDING INSURRECTION

Relentless military attacks on protests have also bred armed insurgency and thousands of rebels and army defectors have formed the Free Syrian Army (FSA) to attack military and police convoys, bases and checkpoints.

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

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 turbulent cities.

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.

(Additional reporting by Ayman Samir and Yasmine Saleh in Cairo and Suleiman al-Khalidi in Amman; Writing by Douglas Hamilton and Erika Solomon; Editing by Mark Heinrich)