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 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 the Nusra Front, 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 U.S., while claiming that they were targeting the Islamic State or the al-Qaida affiliated Nusra Front.

Despite the Russian strikes and although fire by both sides was reported on a number of front lines, the intensity of the fighting appeared to have eased significantly.

On Sunday morning, Russia's Defense Ministry said the truce appeared to be working.

“On the whole, the ceasefire regime in Syria is being implemented,” it said in a statement, according to Russian news agencies.

A cease-fire coordination center at the Russian air base of Khmeimim, in northwestern Syria's Latakia province, accused the rebels of committing nine truce violations in the first 24 hours, singling out an attack by the Islamic State on the Kurdish-held town of Tal Abyad in the northeast as the most serious.

The head of the coordination center, Lt. Gen. Sergei Kurylenko, claimed in televised remarks that Turkey facilitated that attack and that the incursion was “supported by artillery fire from Turkish territory.” He said Russia had lodged a complaint with the U.S. cease-fire coordination center, based in the Jordanian capital, Amman.

Turkey denied the charge, according to military sources quoted by Turkish newspaper Hurriyet.

The Islamic State's incursion into Tal Abyad was the most serious by the militant group in northeastern Syria in more than eight months and was contained only after U.S. warplanes intervened. On Sunday, reports from Tal Abyad said militants were still holed up on the outskirts.