Spiraling sectarian violence killed dozens of people in the troubled Syrian city of Homs on Thursday, casting into doubt prospects that an Arab League peace plan would succeed in tamping down an escalating conflict between pro- and anti-government forces.

Residents and activists described a city descending into war as gunmen on both sides of the divide swarmed into each other’s neighborhoods, abducting and shooting civilians and heralding a worrying twist to the nearly eight-month uprising against the rule of President Bashar al-Assad.

Many of those killed belonged to Assad’s minority Alawite sect, which dominates most of the senior positions in the security forces, according to Homs residents and activists.

There were also reports that members of the majority Sunni sect had been shot down in retaliatory killings by pro-government gunmen circulating in vehicles in Sunni neighborhoods and opening fire at random on civilians.

Confirming the number of deaths was impossible because Syria restricts access to foreign journalists. One of the clauses in the Arab League plan, which Syria endorsed Wednesday, calls for journalists and human rights monitors to be granted unfettered access to the country.

In Washington, Victoria Nuland, a State Department spokeswoman, denounced the killings as evidence of Assad’s “continued history of broken promises.” While applauding the Arab League’s efforts to end violence through negotiations, Nuland said, “We have not seen any evidence that the Assad regime intends to live up to the commitments that it’s made.”

A doctor contacted by telephone at the National Hospital in Homs said that more than 70 bodies had been brought to the facility over the previous 24 hours, most of the victims killed by gunshot wounds. Many appeared to have been disfigured after they were shot, with some of the corpses burned and others hacked with axes, an indicator of the ferocity of the hatred that has been building since the uprising began in March.

“Some of the bodies have been stabbed from their heads to their feet, some have had their eyes taken out and the burned bodies cannot be recognized,” the doctor said, speaking on condition of anonymity because he fears for his safety. “Gunmen who are extremists are touring the city in cars kidnapping people and killing them.”

At the same time, Syrian security forces surrounded and bombarded three key protest flash points in the city, killing at least 16 people, according to activist groups. The assault called into question the government’s commitment to the terms of the Arab League plan, under which Syria agreed to withdraw troops from residential neighborhoods.

Some activists in Homs attempted to downplay the sectarian bloodshed, accusing the Syrian regime of stealing bodies and planting them to cast into disrepute the protest movement and to justify a continued assault on the Sunni neighborhoods where the protest movement has taken hold.

But others said that the killings were real and that they signified the gravity of the divide that has opened between pro-regime members of the Alawite and Christian minorities and the Sunni majority since the uprising began.

“The killing is sectarian, and it is being perpetuated by revenge without moral limits or rules,” said activist Mohammed Saleh, a Homs resident and activist. “The situation is out of hand, and there is nothing that can hold it in check.”

Homs, with its mixed Christian, Alawite, Sunni and Shiite population, is by no means typical of most towns and cities where anti-government protests have been staged in recent months. In many parts of the country where the Sunni population overwhelmingly dominates, the divide has manifested itself more as one between overwhelmingly peaceful demonstrators and security forces loyal to the regime.

But Homs has emerged not only as the most stubbornly resilient of the protest flash points, but also the one most likely to descend into sectarian conflict.

Homs activists say hundreds of soldiers have defected from the army and joined the protest movement, complicating what had begun as a peaceful uprising. But some diplomats and activists suspect that many of those fighting back against the government are civilians who have taken up arms in frustration at the failure of the peaceful protest movement to dislodge the regime.

The surge in violence erupted before the Arab League peace initiative was announced in Cairo on Wednesday evening. On Tuesday, gunmen intercepted a bus carrying regime supporters and fatally shot nine men, mostly Alawites. On Wednesday, the bodies of 11 Sunni workers were found at a factory on the edge of the city.

Shortly before dawn Thursday, the bodies of 27 men, also mostly Alawites, were found dumped at a cemetery near a Sunni neighborhood, according to Saleh, triggering further retaliatory killings throughout the day.

Other activists said they feared that the killings would be used as a pretext by the government to defy the conditions of the Arab League plan and to launch a full-scale attack against protest strongholds. On Thursday evening, government forces used loudspeakers to urge residents in the flash-point Bab al-Amr neighborhood to leave their homes, raising concerns that an offensive was imminent, according to Fady, an activist in Homs who asked that his full name not be used because he fears for his safety.

“We know that the regime is not serious,” he said. “They will never allow the people to take to the streets freely, and they will never comply with any agreement to do so.”

Staff writer Joby Warrick in Washington contributed to this report.