Arab League monitors are expected to visit three key areas in Syria where security forces have violently repressed anti-government protests, as questions mount about the observers' credibility.

The monitors are due Thursday in Daraa, Hama and Idlib on the third day of their mission to check on government pledges to halt a deadly crackdown and release political detainees.

Activists and rights groups say government troops have killed at least 39 people since the monitors arrived Monday, including six people shot in the central city of Hama on Wednesday.

The British-based Syrian Observatory for Human Rights said violence also erupted in several other parts of Syria, including the ambush killings of four soldiers by a group of military defectors.

The continued bloodshed has fueled concerns by the Syrian opposition that the Arab monitors are neither properly qualified nor independent.

Residents of the hard-hit Homs district of Baba Amr refused to allow observers in because they insisted on having army officers with them. The standoff ended when the officers withdrew.

Also Wednesday, Syrian state media said the government released 755 prisoners arrested during the protests. Human Rights Watch said the move was a positive step, but expressed concern that other detainees were being transferred in advance of planned inspections by monitors.

The rights group has accused Syria of moving prisoners to military facilities to hide them from the observers.

The Arab League mission got off to a controversial start Wednesday when its leader, Sudanese General Mohammed Dabi, said monitors did not see “anything frightening” during its initial visit to Homs. He said the city was “quiet” and there were no clashes.

Dabi later said his team will need more time to properly assess the city, which was pounded by heavy government firepower in the days before the visit.

Syrian authorities agreed to the Arab League observers under international pressure and threats of Arab sanctions. The plan requires Syrian authorities to give the monitors freedom of movement except for sensitive military sites.

The United Nations estimates 5,000 people have been killed since March in violence linked to Syria's unrest. Syria says armed terrorists are driving the revolt. It accuses them of killing 2,000 security personnel since March.