ISTANBUL — Tens of thousands of people fleeing a Russian and Syrian offensive in the northwest Syrian province of Idlib have reached the Turkish border this week, swelling the number of displaced people in the rebel stronghold to nearly a half million as another cease-fire all but shatters.

Dozens of people have been killed since Jan. 12, when Turkey brokered a truce with Russia in Idlib, the last opposition-controlled area in war-gutted Syria that has faced intermittent bombing campaigns by Syrian forces and their Russian partners since April. Turkey’s top diplomat this week urged Russia to respect the latest cease-fire, calling its violations “unacceptable.”

Turkish President Recep Tayip Erdogan and his Russian counterpart Vladimir Putin have hammered out a half dozen or so cease-fires for Idlib since last year that have all collapsed. But their continued engagement on Idlib indicates how central the area is in the two countries’ broader relationship, which includes weapons deals and mutual distrust of Washington, as well as attempts at a settlement for the conflict in Libya.

Ankara has shifted its focus to Libya, where it has sent troops (and Syrian opposition fighters) to back the internationally recognized government in Tripoli in exchange for a maritime agreement that could thwart its Mediterranean rivals’ exploitation of natural gas under the seabed. Russia backs Gen. Khalifa Hifter, who is battling to seize the Libyan capital.