China has delayed customs clearance of coal imports from Australia by as long as 40 days, according to a leading industry body, raising speculation that the world’s biggest shipper of the commodity is being targeted at a time of strained relations between the two nations.

Customs officials are taking longer to clear Australian coking coal imports, especially in northern regions, Han Lei, an analyst at China Coal Transport & Distribution Association, said on Wednesday. A few southern ports have started to slow approvals of thermal coal from Australia as well, he said.

“China is sending a warning sign to Australia amid the two countries’ deteriorating relations since last year.”

The measures, in place since last month, come as political tensions simmer between Australia and China, its biggest trading partner. Calls seeking comment from China’s General Administration of Customs weren’t answered.

In recent months, Australia has knocked back a takeover bid by a China-linked company, clamped down on technology investments, and agreed to work with the U.S. on a naval base in Papua New Guinea, a move seen as a direct counter to China’s influence in the Pacific. For its part, China in November launched an anti-dumping probe into imports of Australian barley.

“China is sending a warning sign to Australia amid the two countries’ deteriorating relations since last year,” Feng Dongbin, chief analyst at China Coal Resource, said by phone from Shanxi province. “Overall, China is still determined to control imports this year, but this time it starts with Australia.”

China regularly tweaks import limits to steer its coal market, the world’s largest. Those measures typically target thermal coal as the government tries to ease the country’s reliance on the fuel. The curbs this year are aimed mainly at coking coal. Australia is the world’s largest exporter of this variety, which is used to make steel.

BHP demurs

BHP Group, the largest mining company and an exporter of coking coal to China, said the measures are a reflection of how China wants to manage its consumption of domestic and imported coal and aren’t targeted at Australia. “These pertain to China’s trade with the whole world,” Andrew Mackenzie, chief of the Melbourne-based company, said on Tuesday after the miner posted earnings. “It’s not particularly Australian in its nature.”

“I don’t believe for one moment this is linked to some of the higher level issues of relationships between China and the rest of the world, and including with us,” Mackenzie said.

While China sources almost half of its overseas coking coal from Australia, the shipments make up only a sliver of the country’s needs due to its huge domestic mining industry. In Australia, the raw material accounts for 16 percent of the country’s earnings from commodity exports, according to government figures.

Coking coal futures on the Dalian Commodity Exchange have risen 3.6 percent in the past month.

“The impact on China will be muted,” said Zhao Chaoyue, a research director at Sinowealth Capital Co., adding that steel mills had built up their coal stockpiles before the Lunar New Year break. “When inventories are depleted, there may be some supply tightness as political underpinnings suggest the curbs won’t be resolved soon.”