BEIRUT, Lebanon — A cease-fire negotiated between Syrian rebels and government forces in the country’s war-battered northwest was on shaky ground after intermittent bombing and shelling resumed overnight Thursday in Idlib Province, the country’s last piece of rebel-held territory.

Russia and Turkey, two of the dominant foreign powers in the area, had negotiated the cease-fire between the groups. It took effect on Wednesday, and was aimed at ending more than six weeks of fighting, bombing and shelling as the Syrian government moved to reclaim parts of the northwest.

But amid sporadic breaches of the agreement, the Turkish foreign minister, Mevlut Cavusoglu, told reporters on Thursday that “it is not possible” to call it a “full cease-fire right now.”

“Russia and Iran as guarantor countries should put pressure” on the Syrian government “for the attacks to stop,” Mr. Cavusoglu added. “They should fulfill their responsibilities.”