BEIJING, July 19 (Reuters) - Weeks of heavy rain show no sign of let-up in China where another round of storms hit an eastern city, killing 31 people and injuring 171. The Health Ministry warned citizens that drinking water supplies were under threat.

Large swathes of the country have been hit by severe flooding this summer, killing more than 400 people so far.

The most recent deaths were in Jinan, capital of coastal Shandong province, which received a record of 180 mm (7 ins) of rain within three hours on Wednesday, Xinhua news agency said.

Further rain was also expected to hit the southwestern province of Sichuan where 54 people have died since the beginning of July and 22 were missing, Xinhua said.

"The task for Sichuan to prevent flooding is very heavy -- it could happen any time," Sichuan provincial authorities were quoted as saying.

In the nearby landlocked city of Chongqing, hit by the heaviest rainfall since records began in 1892, 37 people had died and the city and its suburbs had become "isolated islands" as streets flooded, Chinese media said.

The Health Ministry said the floods, and outbreaks of algae on lakes caused by hot weather and pollution, threatened drinking water supplies. It said officials must pay more heed to the problem.

"Since the start of summer, the flood situation has been serious, some areas have had algae outbreaks, and there have been many water pollution incidents," it said in a statement on its Web site (www.moh.gov.cn).

"This has had a large impact on drinking water safety and people's lives," it added. "Drinking water safety has a direct impact on health and social stability and harmony."

In the far western region of Xinjiang, 11 people had died and more than 100 been injured in rainstorms since Saturday, Xinhua said. More than 80 vehicles had been stranded on a highway in neighbouring Tibet where heavy rains caused landslides.

Floods have also caused a 2-billion-strong plague of rats fleeing rising waters in Dongting Lake in Hunan province. The rodents have been blamed for destroying 1.6 million hectares (6,200 sq miles) of cropland and have stoked fears of disease.

Media reports have blamed the rat plague on a lack of snakes, a popular dish in the south, and of owls, used in traditional Chinese medicine.

The Hunan government denied the reports, saying the Dongting Lake area was not an ideal habitat for snakes, Xinhua said.

"The top enemy of the rats are hawks that spend winter in the wetlands around the lake but fly away in the spring," a forestry expert was quoted as saying.

The local government had contained the rat threat and taken effective measures to prevent epidemics, the state agency said.

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