BEIRUT (AP) ― Tanks and troops rolled into southern villages near the heart of Syria’s anti-government uprising Tuesday, while officials in Washington said the U.S. administration is edging closer to calling for an end to the Assad family’s long rule after its violent suppression of the protests.

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

A human rights group reported Tuesday that more than 750 people have been killed in the crackdown.

In Washington, Obama administration officials said the first step in a new American approach toward Syria would be to declare that Assad has forfeited his legitimacy to rule, a policy shift that would amount to a call for regime change.

The tougher U.S. line almost certainly would echo demands for “democratic transition” that the administration used in Egypt and is now espousing in Libya, the officials said. But directly challenging Assad’s leadership is a decision fraught with problems: Arab countries are divided, Europe is still trying to gauge its response and there are major doubts over how far the United States could go to back up its words with action.

The Obama administration’s policy deliberations were occurring against a backdrop of ongoing violence in Syria.

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.

“We urge the Syrian government to stop shooting protesters, to allow for peaceful marches and to stop these campaigns of arbitrary arrests and to start a meaningful dialogue,” State Department spokesman Mark Toner said Tuesday in Washington. He said Assad still had a chance to make amends, but acknowledged “the window is narrowing.”

Toner called the government’s claims of reforms “false,” and demanded that the regime stop shooting protesters even as security forces entered new cities in southern Syria that had been peaceful until now.

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.

If the Syrian government persists with its crackdown on political opponents, the United States could be forced into choosing between an undesired military operation to protect civilians, as in Libya, or an embarrassing reversal that makes it look weak before an Arab world that is on the tipping point between greater democracy or greater repression.

Two Obama administration officials said the U.S. worries about a prevailing perception that its response to Assad’s repression has been too soft, especially after helping usher longtime ally Hosni Mubarak out of power in Egypt and joining the international military coalition to shield civilians from attacks by Moammar Gadhafi’s forces in Libya.

Speaking on condition of anonymity to discuss sensitive internal planning, the Obama officials said Assad has dispelled nearly any lingering hope that he can or will deliver on grandiose pledges of reform he has made since coming to power 11 years ago. After ending decades of martial law last month, his regime renewed its crackdown on peaceful protesters even more aggressively, using live ammunition and arbitrarily arresting thousands of people.

“We’re getting close,” one official said on the question of challenging Assad’s legitimacy, adding that such a step would oblige the U.S. and, if other countries agree, the international community, to act.

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,” said Qurabi, the National Organization for Human Rights in Syria head.

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.

The government’s heavy-handed response has triggered new international sanctions.

President Barack Obama welcomed the European Union’s decision Monday to impose sanctions on 13 Syrian officials, prohibiting them from traveling anywhere in the 27-nation organization. U.S. sanctions target the assets of two Assad relatives and another top Syrian official. But neither EU nor U.S. sanctions affect Assad himself.