HAIKOU, Oct. 22 (Xinhua) -- Saudel, the 17th typhoon to affect China this year, has escalated into a severe tropical storm and is heading toward south China's island province of Hainan, the local meteorological service said on Thursday.

The typhoon was observed over waters 430 km east of Sansha City's Yongxing island in Hainan at 8 a.m. on Thursday, according to the provincial meteorological department.

The department expects the typhoon to move northwestward at a speed of 10 km per hour and continue to gain strength, with a maximum wind force of 118.8 km per hour.

It will move toward the central and northern coasts of Vietnam after brushing past or making landfall in south Hainan on Saturday night, according to the department.

Saudel is expected to bring torrential downpours and strong winds to Hainan from Friday to Sunday. The provincial government has launched a level-IV emergency response in preparation for the storm.

Hainan's emergency management authorities have also warned about potential rain-triggered geological disasters in some areas within the next 24 hours. A total of 13 large and medium-sized reservoirs and 102 small reservoirs in the province are operating at high water levels.