People wearing protection masks walk near the iconic headquarters of China's state broadcaster Central China Television (CCTV) at the Central Business District in Beijing as the capital of China is blanked by heavy smog on Tuesday, Jan. 3, 2017. China's Ministry of Environment says an unspecified number of companies have violated measures meant to reduce smog as the country deals with a phase of particularly noxious pollution. Beijing has been on "orange alert" the second highest pollution alert level since Friday. The alert was originally due to end on Sunday but authorities have extended it a further three days. (AP Photo/Andy Wong)

China's environmental ministry said Tuesday that an unspecified number of companies had violated measures meant to reduce smog during a period of particularly noxious pollution in the country.

The Ministry of Environmental Protection said that 10 inspection teams found companies resuming production despite a government ban and not complying with emission reduction measures.

The announcement came as 24 cities were under a pollution "red alert," the highest warning level in China's four-tiered system. When authorities issue red alerts, some manufacturing companies are required to cut production and heavily polluting vehicles are banned from the roads.

The official Xinhua News Agency said the ministry had given out punishments after finding that more than 500 construction sites and enterprises, including metallurgy, agricultural chemical and steel plants, and 10,000 vehicles had breached pollution response plans. Xinhua gave no details on the punishments.

Beijing has been on "orange alert"—the second-highest alert level—since Friday. The alert was originally due to end on Sunday, but authorities extended it for three days as the smog persisted.

Residents in the capital said the gray air left them feeling depressed.

A Chinese woman wearing a protection mask walks near the iconic headquarters of China's state broadcaster Central China Television (CCTV) at the Central Business District in Beijing as the capital of China is blanked by heavy smog Tuesday, Jan. 3, 2017. China's Ministry of Environment says an unspecified number of companies have violated measures meant to reduce smog as the country deals with a phase of particularly noxious pollution. Beijing has been on "orange alert" the second highest pollution alert level since Friday. The alert was originally due to end on Sunday but authorities have extended it a further three days. (AP Photo/Andy Wong)

"I have a child, but I can only keep him at home and he can't even go out for sunshine, which really worries me," said salesman Zhan Yan.

"The smog has a big negative impact on people's emotions," said another man, Gao Yan, who works for a consulting company. "We really hope we can live with a blue sky and be healthy."

China has long faced some of the worst air pollution in the world, blamed on its reliance of coal for energy and factory production, as well as a surplus of older, less efficient cars on its roads. Inadequate controls on industry and lax enforcement of standards have worsened the pollution problem.

A huge TV screen shows air quality in Beijing as the capital of China is blanked by heavy smog Tuesday, Jan. 3, 2017. China's Ministry of Environment says an unspecified number of companies have violated measures meant to reduce smog as the country deals with a phase of particularly noxious pollution. Beijing has been on "orange alert" the second highest pollution alert level since Friday. The alert was originally due to end on Sunday but authorities have extended it a further three days. (AP Photo/Andy Wong)

A huge TV screen shows forecast of Beijing's air quality as the capital of China is blanked by heavy smog on Tuesday, Jan. 3, 2017. China's Ministry of Environment says an unspecified number of companies have violated measures meant to reduce smog as the country deals with a phase of particularly noxious pollution. Beijing has been on "orange alert" the second highest pollution alert level since Friday. The alert was originally due to end on Sunday but authorities have extended it a further three days. (AP Photo/Andy Wong)

Explore further China starts 2017 engulfed by smog, issues pollution alerts

© 2017 The Associated Press. All rights reserved.