BEIRUT — Syrian security forces backed by tanks and snipers launched a ferocious assault Sunday on defiant cities and towns, killing at least 70 people and possibly many more as the regime raced to crush dissent ahead of Ramadan. Corpses littered the streets after a surge in violence that drew widespread international condemnation.

Estimates of the death toll ranged from around 75 people to nearly 140 on a day when the attacks began before dawn and witnesses said they were too frightened to collect corpses from the streets.

The worst carnage was in Hama, a city with a history of defiance against 40 years of Assad family rule. Hospitals there were overwhelmed with bloodied casualties, suggesting the death toll could rise sharply, witnesses said.

Ramadan, which begins Monday, will present a critical test for the government, which has unleashed deadly firepower since March but still has not been able to put down the revolt. Daily demonstrations are expected to surge during the holy month, when crowds gather in mosques each evening after the dawn-to-dusk fast.

Though the violence has so far failed to blunt the protests, the Syrian government appears to be hoping it can frighten people from taking to the streets during Ramadan.

By mid-morning, the city looked like a war zone, residents said. The crackle of gunfire and thud of tank shells echoed across the city, and clouds of black smoke drifted over rooftops.

An escalation in violence during Ramadan, a time of heightened religious fervor for devout Muslims, would bring a new dimension to the unrest in Syria, which has reached a stalemate in recent weeks. Assad’s elite forces have waged nearly nonstop crackdowns around the country, but new protest hotbeds have emerged — taxing the already exhausted and overextended military.