Strong tropical storm Nalgae headed toward south China on Monday, where typhoon Nesat had just wreaked havoc and left at least four dead.

Nalgae, downgraded from a strong typhoon Sunday afternoon, was located about 450 km southeast of Sanya in the tourist island of Hainan at 9 a.m. Monday and was moving northwest at a speed of 15-20 km per hour, packing winds of up to 108 km per hour in its eye, according to the Hainan Provincial Meteorological Station.

More than 27,000 fishing boats have been called back to harbor in the province and navigation across the Qiongzhou Strait between Hainan and Guangdong Province has been suspended, said Cai Qinbo, director of the station.

It is forecast to slam into Sanya or Wanning in the southern areas of Hainan from Tuesday at noon to evening, or just brush past the island, Cai said.

"No matter whether it makes landfall, it will bring strong winds and heavy downpours to Hainan," he said.

The meteorological station issued a yellow typhoon alert early Monday, he added.

"The key issue now is to ensure the safety of reservoirs, as all reservoirs in the province are running at high water levels," said Yang Yunxian, director of the Hainan Provincial Headquarters for Flood Control,Drought Prevention and Typhoon Precaution.

The Songtao Reservoir in Danzhou, the biggest in Hainan, will open its sluice gate to discharge flood water Monday afternoon, and 15 million cubic meter of water will be released -- the largest amount since 1989.

"After the discharge, the water level in the reservoir, which holds 2.7 billion cubic meter of water now, will drop by one meter," Yang said.

Chen Cheng, vice governor of Hainan, ordered Monday that local authorities must ensure the safety of all fishing boats, crew members, residents in low-lying areas, tourists and reservoirs.

Nalgae will also bring strong winds and heavy rains to south Guangxi Zhuang Autonomous Region in the next two days, and local authorities have been told to guard against geological disasters and ensure the safety of residents.

Nalgae would be the third tropical cyclone to hit Hainan in less than two weeks, after strong typhoon Nesat and tropical storm Hitom.

Earlier, four people died, more than 140,000 were evacuated and 2.49 million were affected in Guangxi in the floods triggered by Nesat, which also toppled houses and damaged farmland.

Direct economic losses were estimated at 2.1 billion yuan (328 million U.S. dollars), and cash crops of sugarcane and banana suffered most, said Wang Chunlin, deputy director of the Guangxi Water Resources Department.

As one of the strongest typhoons to hit Hainan over recent years, Nesat affected 3.77 million people and caused 5.8 billion yuan in economic losses on the island, local authorities said.