GAZIANTEP, Turkey — Syria’s hard-won truce began to fray Sunday, with Russian warplanes resuming airstrikes on towns and villages in the north and fresh reports of artillery fire across several front lines.

The violence came on only the second day of a planned two-week cessation of hostilities, dimming hopes that the calm that took hold Saturday will endure long enough to inject new impetus into a wider peace effort.

The Russian planes, based in northwestern Syria, struck six towns and villages in the provinces of Aleppo, Hama and Idlib early Sunday, according to monitoring and civil defense groups.

The strikes ended a 24-hour suspension announced by the Russian military Saturday to coincide with the launch of the truce. They also appeared to signal a return to attacks that preceded the effort to end the fighting, in which Russia has helped bolster the fortunes of President Bashar al-Assad, a longtime ally.

Russia’s Defense Ministry offered no comment on the strikes, but it had warned Saturday that it reserved the right under the terms of the truce to continue hitting the Islamic State and Jabhat al-Nusra, terrorist groups that are battling the Assad regime.

Russian warplanes have in the past repeatedly struck towns loyal to more moderate rebels, including those backed by the United States, while claiming that they were targeting the Islamic State or Jabhat al-Nusra, which is affiliated with al-Qaida.

The total number of airstrikes was nonetheless significantly lower than in the days preceding the truce, when Russia dropped hundreds of bombs over a wide area of rebel-held territory in an apparent attempt to score last-minute gains. Although artillery and small-arms fire by both sides were reported on a number of front lines, the intensity of the fighting appeared to have eased significantly.