The Syrian military on Sunday escalated its offensive to crush a nationwide revolt, reportedly killing scores of people as it sent tanks and troops to several locations, including the restive eastern city of Deir al-Zour, whose tribes are armed and have close clan ties with their brethren across the border in Iraq.

Residents said tanks began bombarding before dawn and then rumbled into neighborhoods on the outskirts of the desert city. Deir al-Zour has emerged in recent weeks as a stronghold of the protest movement seeking to topple the regime led by President Bashar al-Assad.

Exact casualty figures were hard to obtain because most people were trapped indoors, and residents said bodies were lying unattended in the streets because it was too dangerous for ambulances to reach them. The London-based Syrian Observatory for Human Rights said 42 people were killed; the global advocacy group Avaaz put the toll at 27; an activist in Deir al-Zour said 58 died.

Activists said at least 12 people were killed in similar assaults on towns in the southern province of Daraa, the central province of Homs and the northern province of Idlib. There also were reports of fresh bombardments, additional deaths and mass arrests in Hama, the first target of the offensive, which was launched last week.

“It’s massive, brutal and determined,” said Wissam Tarif, an activist with Avaaz. “They are just killing people everywhere.”

The assault on Deir al-Zour echoed the push launched a week ago against the central city of Hama, where human rights groups say at least 200 people have been killed, leaving little doubt of the government’s intent to use force to crush the nearly five-month-old uprising that has threatened to overturn five decades of Baath Party rule.

Like Hama, Deir al-Zour had effectively fallen under the control of the protesters, who had erected barricades to keep the military out, enabling them to stage massive rallies that routinely drew tens of thousands of people.

Moreover, the city’s remote desert location near Iraq gives it a special sensitivity because many of its residents have family ties to tribes in the Iraqi province of Anbar and, like their kinsmen, typically keep weapons, raising the specter of armed confrontations between the military and anti-government forces.

There also have been unconfirmed reports of army defections in Deir al-Zour, though none seem to have amounted to a significant challenge to the government. An activist contacted via Skype said that about 350 defected soldiers had taken refuge among residents of two western neighborhoods that troops had not invaded.

Although many people in the city keep AK-47s for personal protection, activists were urging them not to turn their weapons against the advancing troops for fear of inviting worse retribution, said the activist, who spoke on the condition of anonymity because he fears for his safety.

“The most important thing is that our revolution remain peaceful,” he said. “Though people keep weapons to defend themselves, they haven’t used them at all.”

The Syrian government has sought to portray the uprising as an armed insurrection by Islamist extremists and “gangs,” and though it is clear that there have been instances of armed confrontation, the protests, for the most part, have been peaceful.

Residents of Hama reported an upsurge in bombardments and mass arrests there Sunday, with troops using tank fire to keep people indoors and then pushing into neighborhoods to hunt down those who had participated in the demonstrations.

An activist contacted by satellite telephone said that the city had come under sustained bombardment throughout the day and that at least 40 people had been killed. More than 500 were detained, he said, as troops searched neighborhoods with lists of names.

“If they go to a house and they don’t find the name on the list, they destroy everything,” he said. “Deir al-Zour was a cover for what has been happening in Hama. In seven days, this is the worst day yet.”

The widening crackdown has drawn mounting condemnation from world powers, which have been reluctant to act against strategically significant Syria, whose alliances with Iran and groups such as Lebanon’s Hezbollah movement give it a linchpin role in the region’s balance of power.

The Arab League, which had remained silent on the violence in Syria, joined the chorus of international condemnation for the first time Sunday, expressing “growing concern and serious distress” at the harsh tactics being used.

Saudi Arabia, one of the league’s most powerful members, announced Monday that it was withdrawing its ambassador from Syria, a significant gesture of disapproval from the autocratic kingdom, which has sought to hold at bay the clamor for reform generated by revolts in the region.

In a written statement read out on al-Arabiya satellite television, Saudi King Abdullah said: “What is happening in Syria is not acceptable for Saudi Arabia. Syria should think wisely before it’s too late and issue and enact reforms that are not merely promises but actual reforms. Either it chooses wisdom on its own or it will be pulled down into the depths of turmoil and loss.

The Arab League also called on Assad to enact political changes, saying there “is still a chance” for reform to work.

But after months of bloodshed in which at least 2,000 people are thought to have died, many Syrians say that it is too late for the government to reform and that, in any case, they would not trust any gestures enough to take them seriously.

On Saturday, the government said it would hold free and fair elections. Hours later, security forces detained a leading democracy activist, Walid Bunni, and his two sons.