BEIRUT—Syrian government forces backed by Russian airstrikes pushed to make strategic gains on Wednesday, intensifying attacks on rebels the day after the U.N. set a date next week for the resumption of peace talks.

Still, neither the Syrian government nor opposition formally indicated on Wednesday that it saw the increasing violence as jeopardizing the truce in place since Saturday. Both sides appeared wary of playing the spoiler in the first serious attempt to halt the bloodshed.

The stepped-up government offensive focused on hilltop towns in the northwestern province of Latakia and the countryside north of Aleppo, Syria’s largest city.

Activists also reported bombardment in the countryside around the central city of Homs and across Idlib, a province bordering Turkey that is mostly controlled by rebels, dominant among them the Syrian al Qaeda offshoot Nusra Front.

In Washington, senior U.S. administration officials said Wednesday a decision by all parties to abide by the cease-fire wouldn’t alone be enough for it to endure. Efforts to forge a political resolution to the Syrian conflict would also have to progress, they said.