BEIRUT (AP) -- More than 750 civilians have been killed in Syria since an uprising against President Bashar Assad's regime began in mid-March, a human rights group said Tuesday as the government pressed its efforts to end the nationwide unrest.

Ammar Qurabi, who heads the National Organization for Human Rights in Syria, told The Associated Press that the group has lists of names, age, cause of death and the area where 757 people died. By comparison, fact-finding investigators said at least 219 people were killed in Tunisia and at least 846 in Egypt in the uprisings that succeeded in toppling their longtime autocratic rulers.

Qurabi said thousands of Syrians have been detained in the past two months, including about 9,000 who are still in custody.

Assad has dispatched troops and tanks to many areas to crush the seven-week uprising that poses the most serious challenge to his family's 40-year rule.

Qurabi spoke to The Associated Press as Syrian forces conducted operations in several hot spots throughout the country.

Earlier Tuesday, an activist reported that Syrian troops backed by tanks entered several southern villages near the flashpoint city of Daraa. The activist said heavy gunfire was heard when the troops entered Inkhil, Dael, Jassem, Sanamein and Nawa after midnight, but it was not clear if there were casualties.

He spoke on condition of anonymity for fear of government reprisals.

In recent weeks, army troops carried out an 11-day operation in Daraa that killed more than 80 people, residents and activists said. The city, near the Jordanian border, has been cut off for the past two weeks.

Assad's regime appears determined to crush the uprising by force and intimidation, despite rapidly growing international outrage and the high death toll.

Human rights activist Mustafa Osso said some of the most intense operations were taking place in the Damascus suburb of Maadamiyeh, which has been sealed for days. He said telecommunications have been cut and checkpoints were preventing anyone from entering or leaving the area.

"Maadamiyeh is isolated from the rest of the world," Osso said.

The army was also conducting operations in the coastal city of Banias, the central city of Homs, and the northern city of Deir el-Zor, Osso said.

"Any area where there are demonstrations, the government is sending the army," he said.

Rights groups said hundreds of people have been detained over the past few days in different areas.

Qurabi said authorities on Tuesday released Hassan Abdul-Azim, at prominent opposition figure who heads the outlawed Arab Socialist Democratic Union party. He had been in custody since April 30.

Also Tuesday, a religious leader who resigned last month in disgust over the killings of protesters in the province of Daraa withdrew his resignation. Sheikh Rizq Abdul-Rahim Abazeid, mufti of the Daraa region, resigned April 23 after shootings by security forces killed scores of people.

"I cannot tolerate the blood of our innocent sons and children being shed," Abazeid told The Associated Press after stepping down. But in an interview with a Syrian satellite TV channel, Abazeid said his resignation was a result of "severe pressure and intimidation," including death threats.

In neighboring Jordan, Jordanian taxi driver Shadi Zouebi said he was mistreated while in Syrian custody for three weeks. The man, who ferries passengers and goods between Jordan and Syria, refused to provide details.

Jordanian newspapers reported that Syrian intelligence agents pushed his head into a toilet and beat him so badly that he considered committing suicide.

Zouebi is the first of 20 Jordanians held by Syria to be freed under negotiations by Jordan's Foreign Ministry.

The government's heavy-handed response to the widespread protests has triggered new international sanctions.

On Monday, the European Union imposed an arms embargo. The measure, which followed U.S. sanctions, also prohibits 13 Syrian government officials from traveling anywhere in the 27-nation EU and freezes their assets.

Trying to increase the pressure on Assad's regime, the United States has imposed sanctions targeting three senior Syrian officials as well as Syria's intelligence agency and the Revolutionary Guard in Iran, a key Syrian ally.

The United Nations said Monday that a planned humanitarian mission had not been allowed access to Daraa.

U.N. humanitarian chief Valerie Amos said the mission was postponed until later this week, and she was trying to find out why the initial plan was thwarted.

The unrest gripping Syria was triggered by the arrests of teenagers caught scrawling anti-government graffiti on walls in Daraa. Despite boasts by Assad that his nation was immune from the kind of uprisings sweeping the Arab world, protests against his rule quickly spread across the country of 23 million people.

Assad, who inherited power from his father in 2000, has blamed "armed thugs" and foreigners. The regime has hit back at protesters with large-scale military operations.

Syria has also banned foreign media and restricted access for reporters to many parts of the country, making it difficult to independently confirm witness accounts of the violence.

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Associated Press writer Sameer N. Yacoub in Amman, Jordan contributed to this report.