Turkey’s military intervention in the nine-year-old Syrian conflict took a further deadly turn when regime forces killed five Turkish soldiers in the rebel-held northwestern province of Idlib today.

Turkey’s Ministry of Defense said another five Turkish troops were wounded when Syrian government forces opened fire on them. The attack on the Taftanaz military base, which the Turkish army took over from the forces of Syrian President Bashar al-Assad last week, came as Turkey poured fresh troops and equipment into the province following Turkish President Recep Tayyip Erdogan’s vows to retaliate for a separate attack seven days ago. Eight Turkish soldiers died in that attack.

Erdogan said Damascus must pull back its forces from new territory it has captured in recent weeks or face a full-scale Turkish offensive. But regime forces kept advancing over the weekend, entering the strategic town of Saraqeb.

The escalation highlights Turkey’s failure to persuade Russia, which is backing the regime’s ongoing offensive to wrest Idlib from the jihadi Hayat Tahrir al-Sham, to pause hostilities. A Russian delegation led by Deputy Foreign Minister Sergey Vershinin and special envoy on Syria Alexander Lavrentiev has been in Ankara since Saturday and met today with presidential spokesman Ibrahim Kalin.