China's environment ministry has ordered cleanups at 20 chemical and petrochemical enterprises, including CNPC and units of Sinopec, after they were found to pose serious safety threats, domestic media reported on Thursday.

The State Environmental Protection Administration (SEPA) also decided to stop or postpone approval for projects at 44 sites with a total investment of 149.5 billion yuan ($18.7 billion) because their locations were considered unsafe.

"SEPA will be responsible for directing the rectification of the 20 projects that inspections found posed hidden environmental risks," the Beijing News reported.

SEPA's inspection of more than 100 sites comes months after an explosion at a chemical plant in the northeast poured benzene compounds into the Songhua River, poisoning the source of drinking water for millions.

Twelve of the 20 projects inspectors found to pose hazards were located along China's two main riverways, the Yellow and Yangtze, and had "serious hidden dangers," SEPA said.

The targets listed by the Beijing News included China National Petroleum Corp., the country's largest oil producer and parent of listed PetroChina.

Subsidiaries of top refiner Sinopec Corp. were also on the list.

The 20 plants will invest more than 1.6 billion yuan to improve environmental protection facilities, the China Daily quoted SEPA's deputy head Pan Yue as saying, but he added such measures were only temporary solutions.

"The right way to stop improper industrial distribution is to ensure environmental impact assessment for newly launched projects and promote it in policy-making," Pan said.

One of the plants singled out, a branch of CNPC in the southwestern province of Sichuan, planned to build an ethylene production factory along a main tributary of the Yangtze in the midst of flood land, without adequate measures to protect the water, the newspaper said.