Syria's state-run TV says authorities have released 552 prisoners allegedly involved in anti-regime activities during the nine-month uprising against President Bashar Assad.

It is the second group of prisoners to be released in a week under an Arab League plan meant to stop the government's crackdown on dissent, which has claimed thousands of lives. Besides halting the violence, the plan calls for the release of all political prisoners. About 3,500 were freed on Tuesday.

Following Thursday's TV report, activists said Syria was still holding at least 25,000 political detainees.

The figures are impossible to verify, with Assad's regime continuing to bar almost all foreign journalists or human rights groups from entering Syria even though that is one of the stipulations in the Arab League plan. Assad agreed to the plan on Dec. 19.

The Arab League has acknowledged that its plan has failed to stop violence. Activists have reported nearly 400 deaths since the monitors began work last week.

The prisoner release announcement Thursday came a day after French President Nicolas Sarkozy issued a new call for Syrian Assad to step down.

The Arab League said some progress was seen in Syria by the team of monitors who began working last week, but noted that the mission was still in its early stages.

Massacres 'arouse disgust'

Sarkozy insisted that Assad "must leave power."

"The massacres being committed by the Syrian regime rightly arouse disgust and revolt in the Arab world, in France, in Europe and everywhere in the world," Sarkozy said in an address at a navy air base in Lanveoc-Poulmic, France.

The UN estimated several weeks ago that more than 5,000 people have been killed by Syrian security forces in the crackdown on the anti-government protests that began in March. Since that report, opposition activists say hundreds more have been killed.

The violence has drawn broad international condemnation and sanctions, but Assad remains defiant.

The Arab League sent in the about 100 observers a week ago to verify Syria's compliance with the organization's plan that requires the regime to remove security forces and heavy weapons from city streets, start talks with opposition leaders and free political prisoners.

Syria agreed to the plan, intended to halt the crackdown, on Dec. 19.

Death figure tallies differ

However, an activist group on Thursday accused the Syrian regime of torturing hundreds of people to death in overcrowded prisons, jails and illegal detention centres across the country since the uprising began.

Avaaz, an online global activist group, issued a report saying 617 people have been confirmed killed under torture by Assad's forces as they cracked down on the revolt.

"Assad's henchmen have tried to break the pro-democracy movement in these torture chambers, but brave Syrians are still standing up for their rights," said Stephanie Brancaforte, campaign director at Avaaz.

The alleged torture victims are among 6,874 people killed since the uprising began, Avaaz said, giving a significantly higher toll than the UN's estimate of 5,000.

Avaaz said it verified each death by three independent sources, including a family member of the deceased and the imam who performed the funeral procession. The group said it works with a team of 58 human rights monitors in Syria.

The group encouraged Arab League monitors in the country to "visit these torture chambers and ensure the regime immediately end these atrocities."