Chinese Minister of Environmental Protection Chen Jining answers questions at a press conference on how to promote environmental protection on the sidelines of the fourth session of China's 12th National People's Congress in Beijing, capital of China, March 11, 2016. [Photo/Xinhua/Zhao Yingquan]?

Environmental authorities across China fined polluters 4.25 billion yuan (654 million U.S. dollars) in 2015, an increase of 34 percent over 2014, Minister of Environmental Protection Chen Jining said on Friday.

The authorities delivered more than 97,000 punishment notices to polluters, Chen told a press conference on the sidelines of the annual session of the National People's Congress (NPC).

"They also worked with police and judicial agencies so that serious offenders not only faced administrative punishment but also criminal penalties," he said.

Polluters involved in 2,079 cases were placed under police detention and those in 1,685 cases were prosecuted.

About 1.77 million enterprises were inspected, with 191,000 of them punished, 20,000 closed and 34,000 suspended production.

"I would like to stress that severe punishment is not an end but a means to have enterprises understand the importance of abiding by the law," Chen said.

The ministry also stepped up supervision of local governments, inspecting 33 cities and interviewing 15 chief city officials.

China's revised environmental protection law took effect in January 2015.