More than 1,000 people have died or disappeared in severe flooding in China so far this year, and the heaviest rains are still to come, a senior official warned Wednesday.

This year's floods, which have caused tens of billions of dollars in damage already, have exacted the highest death toll since 1998, which saw the highest water levels in 50 years.

The waters have killed 701 people and left 347 missing, officials said.

With the typhoon season rolling in, Liu Ning, general secretary of the government's flood prevention agency, told a news conference authorities must ramp up preparations.

"Since 60 to 80 per cent of the annual rain level occurs in June, July and August, we should be prepared to prevent and combat potential disasters," Liu said.

Tropical storm Chanthu is expected to hit China's southern island of Hainan and Guangdong province this weekend. Six to eight typhoons are expected this year.

Already, three-quarters of China's provinces have been plagued by flooding and 25 rivers have reached record-high water levels, Liu said.

Flooding, particularly along the Yangtze River basin, has overwhelmed reservoirs, swamped towns and cities, and caused landslides that have smothered communities, including toppling 645,000 houses. The Three Gorges Dam faced its highest levels ever this week and water breached the massive dam.

"Although water levels in the upper stretches of the Yangtze River have surpassed that of 1998, the flood situation is still not as severe because the Three Gorges Dam has played a key role in preventing floods along the river this year," Liu said.

The overall damage totals about $21 billion US, Liu said.