A truck went off the road and landed in a ditch between Aytos and Ruen, near the turnoff for the village of Zetyovo.
Three ambulance crews are at the scene, and police are securing the area.
Traffic on the third-class road between Aytos and Dulgopol is restricted, the Road Infrastructure Agency says. Drivers heading to Burgas should detour through Yabalchevo, Prosenik, Orizare, Tankovo and Sunny Beach; those going to Sliven should take the route via Vresovo, Lyulyakovo, Saedinenie, Prilep and Karnobat.
The agency is asking drivers to take extra care and slow down. The detours will stay in place until crews clear the crash site.