SHANGHAI (Reuters) - The Yangtze River Delta region near Shanghai saw a key smog measure rise by a fifth in January, making it more polluted than Beijing and raising fears that the pollution crackdown in the north has forced heavy industries to head south.

FILE PHOTO: A man wearing a face mask rides a bicycle on a bridge in front of the financial district of Pudong covered in smog during a polluted day in Shanghai, China November 22, 2017. REUTERS/Aly Song

Though concentrations of small, breathable particles known as PM2.5 fell 17.9 percent year on year to 64 micrograms per cubic meter in 338 cities nationwide in January, the Yangtze River Delta registered an average of 72 micrograms, up 20 percent, according to official data published on Monday.

The Pearl River Delta region around Hong Kong saw PM2.5 rise 3.9 percent in January from a year ago, data from the Ministry of Environmental Protection (MEP) also showed.

“The past three months mark the first time on record that Shanghai and Guangzhou registered worse particle pollution levels than Beijing - going back to 2001,” said Lauri Myllyvirta, a Beijing-based campaigner with environmental group Greenpeace.

Weather played a role, but the spike in emissions in the Yangtze river region was also due to a rebound in polluting industries like steel, especially after production was curbed in northern regions, Myllyvirta said.

Xian, home of China’s Terracotta Army in the country’s northwest, was the worst-performing city during January.

China saw unusually high levels of PM2.5 in the first two months of 2017, when near-record pollution throughout northern China prompted dozens of cities in the region to declare a “red alert” to impose emergency traffic restrictions and close factories and schools.

In a bid to meet annual targets after the early 2017 spike, 28 northern cities pledged to cut PM2.5 by 10-25 percent from October 2017 to March 2018, forcing them to curb industrial output, reduce vehicle traffic and slash coal consumption.

The measures cut PM2.5 emissions by an average of 33 percent to 77 micrograms from October last year to the end of January, the environment ministry said in a separate notice on Monday.

It said Beijing cut pollutants 58.3 percent over the period, while Shijiazhuang, capital of Hebei province, registered a 52.4 percent decline over the four months.

Greenpeace’s Myllyvirta said it was likely the tough restrictions in northern China shifted more industrial production to central China.

Jiangsu province to Shanghai’s north is China’s second-biggest steel producer behind Hebei, but has not been subject to any special pollution measures over winter. Jiangsu’s Xuzhou, a big steel production zone, was identified as China’s most polluted city in December.

While China has claimed initial success in its four-year “war on pollution”, average PM2.5 in January remained far higher than the state standard of 35 micrograms. The World Health Organization recommends no more than 10 micrograms.

China’s anti-smog campaign has also not been without controversy, with businesses complaining that Beijing’s “one size fits all” approach has harmed their interests.

An over-ambitious attempt to convert millions of households from coal to natural gas heating has also proved a major challenge, with many villages left without gas supplies during parts of the freezing winter.