More tanks and troops rolled into southern villages Tuesday near the heart of Syria’s anti-government uprising, with activists saying the regime has isolated parts of the country. A human rights group reported that more than 750 people have been killed in a crackdown on seven weeks of unrest.

The military has been sealing off various areas of Syria and conducting house-to-house raids in search for people whose names are on wanted lists, with many fleeing cities and towns for fear of detention by the regime of President Bashar Assad, activists say.

Intense military operations have taken place in the Damascus suburb of Maadamiyeh, which has been sealed off for days, said human rights activist Mustafa Osso. He said communications 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 also was 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.

Another activist said troops backed by tanks entered southern villages near Daraa, the city where the uprising began in mid-March. 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, according to the activist, who spoke on condition of anonymity for fear of government reprisals.

Thousands of Syrians have been detained in the past two months, including about 9,000 who are still in custody, said Ammar Qurabi, who heads the National Organization for Human Rights in Syria.

Qurabi told The Associated Press that the group has documented the deaths of 757 people.

The death toll has increased as authorities intensify their crackdown on the uprising, which poses the most serious challenge yet to the Assad family’s 40-year rule in Syria.

Assad has said Syria was immune from the pro-democracy movements sweeping the Arab world that ousted leaders in Tunisia and Egypt. Protests of his rule, however, have spread rapidly across the country of 23 million people.

His regime appears determined to crush the popular revolt by force and intimidation, bringing increasing international outrage.

U.N. humanitarian chief Valerie Amos said the deployment of tanks and reports of shelling of residential areas in Syria are “alarming.” Her remarks came during a Security Council debate on the United Nations’ responsibility to protect civilians in armed conflict.

Syria has a history of deadly crackdowns. Assad’s father, Hafez Assad, crushed a Sunni uprising in 1982 by shelling the central town of Hama, killing 10,000 to 25,000 people, according to Amnesty International estimates. Conflicting figures exist and the Syrian government has made no official estimate.

Osso said the two deadliest areas since the uprising began were the southern province of Daraa and the central region of Homs, although he did not have figures for each area.

“In some areas prisons and security offices are full. They are putting detainees in schools, football fields or government buildings,” Qurabi said.

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.

Rights groups said hundreds of people have been detained in the past few days in different areas. Qurabi said 450 people have been detained in Banias alone in the past three days.

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

Later, Osso said authorities freed leading opposition figures Kamal Sheikho, George Sabra and Fayez Sara, a well-known writer and journalist.

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

The government’s heavy-handed response 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.

The United States has imposed sanctions against three senior Syrian officials as well as Syria’s intelligence agency and the Revolutionary Guard in Iran, a Syrian ally.

In an apparent response to the crackdown, Western diplomats said Kuwait will replace Syria as a candidate for a seat on the U.N.’s top human rights body.

The U.N. said Monday that a humanitarian mission had not been allowed access to Daraa.

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.

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 regions, making it difficult to independently confirm witness accounts of the violence.