ISTANBUL — Dozens of people were killed in airstrikes in the Syrian province of Idlib this week in what the United Nations called a “worsening nightmare,” as a truce negotiated between Russia and Turkey for the last rebel stronghold threatens to shatter.

Turkish President Recep Tayyip Erdogan won a reprieve from a Russian-backed Syrian government onslaught on the northwestern province in September 2018. Now his focus appears to have shifted to the other side of Syria amid a Turkish troop buildup that portends an incursion east of the Euphrates River against a Kurdish militia Ankara deems a terrorist group.

In a series of raids on Idlib, at least 59 civilians were killed and more than 100 people wounded on Monday, according to a UN statement that called for an immediate halt in the attacks to protect the three million civilians living in the province.

The Syrian Observatory for Human Rights said Syrian President Bashar al-Assad’s forces as well as their Russian allies carried out the bombardments from the air and ground as they push to retake Idlib in what could be the last — and bloodiest — battle in the country’s eight-year war.