(Recasts, adds details of missing fishermen in paragraphs 4,5)

BEIJING, April 19 (Reuters) - Typhoon Neoguri has weakened into a tropical storm after slamming into the southern Chinese island province of Hainan, with 18 fishermen reported missing, state media said on Saturday.

The storm forced the cancellation of 76 flights, caused a power black-out in Hainan's northeastern city of Wenchang, and is now moving into Guangdong province, Xinhua news agency said.

Guangdong flood control authorities said residents in low-lying areas and dangerous houses had all been evacuated to safety, it reported. About 20,000 boats in Guangdong had returned to port.

Though 38 fisherman who had sought shelter in reefs around the Paracel Islands in the South China Sea had been rescued, another 18 from the same group were still missing, Xinhua said.

"All the three boats were damaged and the 38 rescued fishermen swam to reefs and stayed there until being found by a rescue ship," Zhang Jie, vice director of the Hainan Maritime Safety Administration, was quoted as saying.

Chinese scientists have blamed global warming for increasing weather extremes, including devastating typhoons, snow storms, floods and drought, which they say are likely to get worse.

Typhoons, known in the West as hurricanes, are cyclonic storms which draw strength from the warm waters of the South China Sea and regularly target the Philippines, Japan, China, Taiwan and Hong Kong over the summer, sometimes with catastrophic effect.

The typhoon season usually starts in May. (Reporting by Ben Blanchard; Editing by Jeremy Laurence)

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