Severe flooding across southern China has forced the world's largest power plant to slash capacity on Tuesday, delayed grain on barges and damaged farms along the Yangtze River, as the death toll rose to 56 and economic costs hit almost $4 billion.

Heavy rainfall, mudslides and hail caused by the annual rainy season has killed 56 people and 22 people were missing across 11 provinces and regions as of Tuesday morning, according to the Ministry of Civil Affairs.

More than 750,000 hectares (1.85 million acres) of crops have been damaged and direct economic losses totaled more than 25.3 billion yuan ($3.72 billion), it said.

The government said it had disbursed 700 million yuan ($103 million) in emergency aid to four flood-hit provinces - Zhejiang, Jiangxi, Hunan and Guizhou.

Rain in the southern provinces is expected to ease in the coming days, but weather forecasters predict downpours will move to the southwestern province of Sichuan.

In what analysts said was a move unprecedented in its scale, the Three Gorges and Gezhouba, two of China's top hydropower plants, closed as much as two-thirds of their capacity to avert flooding further downstream on the Yangtze River.

The move stoked concerns about electricity supplies from China's second-largest power source as a heatwave continued to scorch northern parts of the country, raising the export prices of coal, the fuel the country uses to produce most of its power.

Coal from Australia's Newcastle terminal rose to its highest since April, with mining outages tightening supply amid strong northern hemisphere summer demand.

The annual rainy season, which arrived in the second half of June, has hit southern Hunan province, one of the nation's largest hog and freshwater fish producers, the most.

High water levels on the Yangtze, Asia's largest river, also slowed barges carrying grain from northern ports to the south, spurring a rise in freight rates and physical corn prices in some regions, analysts and corn buyers said.

Zhang Yi, a purchase manager at a feed producer in Hunan, said he had three ships carrying about 5,000 tonnes of corn stuck on waterways near the port of Changsha, the capital of Hunan, since Friday.

-REUTERS