BEIRUT – Syrian forces on Thursday renewed attacks against rebel strongholds in the nation’s two largest cities, highlighting the determination of President Bashar Assad’s government to crush resistance in Damascus and the northern city of Aleppo.

For a second consecutive day, opposition activists said, the military used mortars and airstrikes in Daraya, a suburb south of Damascus, the capital.

“They are shelling it at an insane pace,” said Moaz Shami, a Damascus-based activist.

About two dozen civilians have been killed in the area, the opposition said. As is often the case in Syria, there was no independent corroboration of the fighting or death toll.

The reported onslaught appears to be part of a concerted government effort to root out insurgents and sympathizers throughout the greater Damascus area. The government has already swept through many city districts in a bid to crush rebels who rose up there last month.

The rebel offensive in the capital raised the specter that Damascus could fall into rebel hands or at least veer largely out of government control. Instead, Assad’s security forces appear to have beaten back rebel forces.

But opposition forces in Damascus and elsewhere say they have not been vanquished in a rebellion that has now extended to much of the country, posing a challenge for a military that has suffered major casualties and many defections. Although the recent focus has been on Damascus and Aleppo, fierce clashes continue in the central city of Homs. The eastern city of Dair Alzour remains a battleground, as are parts of the southern province of Dara, where the uprising began almost 18 months ago.

More than 200 miles north of the capital, fighting continued in Aleppo. Four weeks ago, rebel forces from outside the city occupied several Aleppo neighborhoods, vowing to seize the nation’s commercial hub. Some envisioned Aleppo becoming a kind of rebel haven and capital, just as Benghazi in Libya became the unofficial rebel capital last year during the revolt in that country.

But, as in Damascus, the rebel offensive in Aleppo has met stiff resistance from government troops who have a distinct advantage in firepower compared with rebels armed mostly with rifles, rocket-propelled grenades and homemade bombs. Strikes from helicopter gunships and jet fighters have now become a regular feature of the government counter-assault in Aleppo.