REYHANLI, Turkey—At minimum 29 Turkish troopers have been killed in northwestern Syria on Thursday, Turkish officials mentioned, plunging Turkey further into the war there, as a Russian-backed Syrian navy offensive sought to reclaim the previous rebel stronghold.

The troopers have been killed by Syrian routine forces, according to the governor of Turkey’s Hatay province, which borders Syria. Turkish officials mentioned another 36 have been wounded.

As…