REYHANLI, Turkey—At the very least 29 Turkish troopers were killed in northwestern Syria on Thursday, Turkish officers reported, plunging Turkey deeper into the war there, as a Russian-backed Syrian military services offensive sought to reclaim the very last rebel stronghold.

The troopers were killed by Syrian routine forces, in accordance to the governor of Turkey’s Hatay province, which borders Syria. Turkish officers reported a further 36 were wounded.

As…