BEIRUT, Lebanon — The Syrian military defied growing condemnation and initiated another assault on the country’s most restive locales on Sunday, deploying dozens of tanks and armored vehicles into parts of a city in eastern Syria that it had long feared provoking, activists and residents said. Dozens were killed, they said, and thousands had fled the city.

The attack before dawn on the eastern city, Deir al-Zour, came exactly a week after Syrian forces attacked Hama, a city in central Syria that had largely wrested itself from government control this summer. Like Hama, Deir al-Zour, in Syria’s oil- and gas-producing region, had been the scene of mass protests, with hundreds of thousands in the streets. But the military, wary of the city’s powerful and well-armed extended clans, had mostly stayed on the outskirts.

Together, the two cities — Syria’s fourth and fifth largest — have been the most defiant in a five-month uprising against four decades of rule by the Assad family. After a week of strong rebukes by a chorus of international voices, from the United Nations to the pope, the renewed assault confirmed what many saw as the determination of President Bashar al-Assad’s government to keep power through violence.

By the count of some human rights groups, more than 2,000 people have been killed in the crackdown so far.