SHANGHAI (Reuters) - A major smog indicator in China’s heavy industrial Beijing-Tianjin-Hebei region rose by more than a quarter in March, the Ministry of Ecology and Environment said on Friday, raising concerns that pollution is increasing with the end of winter output restrictions.

A building under construction is seen amid heavy smog in Beijing's central business area, China April 2, 2018. REUTERS/Jason Lee

Concentrations of particulate matter of less than 2.5 microns, known as PM2.5, climbed to 80 micrograms per cubic meter in the region last month, an increase of 27 percent from a year ago, the ministry said.

The region includes China’s capital and Tangshan, the world’s largest steel producing city.

A six-month anti-pollution campaign that ended on March 15 required 28 Chinese cities to reduce PM2.5 levels by as much as 25 percent by slashing industrial output, restricting traffic and cutting coal use.

During the first quarter, the levels of PM2.5 in the Beijing-Tianjin-Hebei region, which has been under heavy political pressure to tackle pollution, dropped 22.1 percent to 74 micrograms per cubic meter, down from near record highs in the same period of last year.

That indicates the output cuts were working as intended but their expiration has increased concerns that pollution levels are rebounding as factories, freed from restrictions, ramp up output.

Nationwide, during the first quarter, PM2.5 concentrations fell 9.7 percent from a year ago to 56 micrograms per cubic meter in the 338 cities monitored by the ministry. However, in March, average concentrations climbed 2.1 percent from the year before to 48 micrograms per cubic meter.

Seven of China’s 10 most polluted cities in March were in Hebei province, China’s biggest steel producing region. Tangshan and the city of Handan have already decided to extend the winter curbs until November.

However, Xuzhou, an industrial city in eastern China’s Jiangsu province, was also on the list of the 10 smoggiest cities in March and during the first quarter, a sign that China’s smog is shifting from its northern heartlands as a result of the crackdown.

The Yangtze river delta region, which comprises Jiangsu, Zhejiang and Anhui provinces and the city of Shanghai, saw PM2.5 levels fall 4 percent in March, but average levels rose 1.8 percent in the first quarter as a whole.

China is currently working on a new 2018-2020 action plan to tackle pollution, and experts have urged the government to introduce more targeted measures to tackle rising ground-level ozone, which has become a growing urban health threat.

Chinese authorities are working to reduce PM2.5 concentrations because the small size of the particles allows them to be inhaled deep into the lungs and their accumulation eventually can cause lung and heart disorders.