BELGRADE- Serbia has ordered China's Zijin Mining Group to halt work at a shaft at the country's only copper mine and to complete a waste water treatment plant after it failed to comply with environmental standards, the mining and energy minister said on Wednesday.

Zijin became Serbia's strategic partner in the RTB Bor copper complex which includes the Jama mine, pledging to invest $1.26 billion in return for a 63% stake.

Minister Zorana Mihajlovic said authorities in late March ordered Zijin's Serbia-based unit to halt work on its Jama copper mining shaft.

"The state has ... responded to the complaints of citizens living in the vicinity of the Jama mine," Mihajlovic said in a statement.

In a statement later on Wednesday, Zijin's unit in Serbia said it had halted construction of ventilation shafts for the Jama mine located in the town of Bor, as ordered by authorities, after residents complained about noise levels.

"The exploitation of copper in the Jama mine is ongoing, deep under ground," it said.

Mihajlovic said the Zijin unit had also been ordered to urgently complete a waste water treatment plant and stop polluting the River Pek, a tributary of the Danube.

"They have until April 30 to eliminate all irregularities," she said.

China has invested billions of euros in Serbia, mostly in the form of soft loans to finance highway and energy projects.

Last week, Zijin's unit in Serbia said it plans to invest $408 million in 2021 to overhaul, expand and improve environmental standards at its mines and a smelter at the Bor complex. 

In its 2019 report, the Global Alliance on Health and Pollution watchdog said Serbia was Europe's most polluted country.

Last Saturday thousands rallied in the capital Belgrade to protest against a lack of government action to prevent pollution mainly caused by industry and mining.

(Reporting by Aleksandar Vasovic; editing by Daria Sito-Sucic and Mark Potter) ((aleksandar.vasovic@thomsonreuters.com; +381113044904;))