"China is a valued partner of Australia and we trust that our free trade agreement commitments to each other will continue to be honoured," Mr Birmingham said in a statement.

Analysts in China said Australian coal had been singled out with imports from other sources, including Mongolia, Russia and Indonesia, unchanged or increasing. While observers are divided on whether a slowdown is due to diplomatic tensions or aimed at propping up domestic prices, the country's top-circulating newspaper said on Thursday the delays were another sign that Beijing was not happy with Australia.

​"Can't even sell coal to China," the Cankaoxiaooxi newspaper, which is published by state news agency Xinhua, said in a story quoting an academic linking restrictions on Australian coal imports imposed last month to the Huawei dispute. The time taken to clear Australian coal through customs has doubled to around 45 days since January, which meant some traders had stopped ordering the commodity in fear of more delays.

Any formal restrictions by China would be the latest blow to the coal industry. Australias' biggest coal producer Glencore this week capped coal production at 2019 levels in what is seen as victory for the global anti-coal lobby. BHP Billiton chief executive Andrew Mackenzie said approvals to import coal into China had stretched out from 25 to 40 days, but dismissed the idea this was linked to Australia's political relationship with China.

Other analysts also said the restrictions were designed to boost domestic coal prices rather than disrupt flows of one Australia's biggest exports to China. Despite this, the latest round of commentary in China's state media this week was more critical of Australia than it has been for some months.

An editorial in the influential China Daily said Prime Minister Scott Morrison was "irresponsible" for this week alluding to the idea that Beijing was behind a cyberattack on the Australian parliament. Mr Morrison on Monday attributed an attack on the Liberal, National and Labor parties' networks to a foreign government without naming China.

"No matter whether he was assigning the malevolent acts to China or another country, it is irresponsible of him to cast aspersions in this way," the China Daily said.

"Certainly it seems that when it comes to China, many politicians, have forgotten their professional ethics, and appear to have no compunction about letting their tongues wag maliciously about a sovereign nation and its companies."


The hawkish Global Times newspaper also said on Thursday that a drop in Chinese investment in Australia was the result of a "Cold War mentality" which had resulted in restrictions on capital from mainland China and Hong Kong. Foreign Investment Review Board (FIRB) figures out this week showed the US surpassed China as the largest source country for approved proposed investment last year for the first time since 2012-13.

While the formal rhetoric from Beijing towards Australia has been positive for the past six months, fund managers have warned any diplomatic dispute which threatened commodity imports posted the biggest economic risk for Australia.

Simon Birmingham, the Minister for Trade, Tourism and Investment Simon Birmingham said: "I'm aware of unconfirmed and unsourced media reports and have asked our ambassador in Beijing to urgently clarify their veracity.

"We continue to engage closely with industry on matters of market access. I met with the Minerals Council of Australia this week to discuss market access issues and our related representations to Chinese authorities.

"Australia is, and will continue to be, a reliable supplier of resources around the world. Australia's exports of coal to China in the 4th quarter of 2018 were higher in volume and value than the same period in 2017.

"China is a valued partner of Australia and we trust that our free trade agreement commitments to each other will continue to be honoured."

The Australian Financial Review first reported in September that China had introduced unofficial restrictions on coal imports to encourage local power stations to buy more domestic coal. China is believed to have ramped up those restrictions in January. Analysts at the Mysteel consultancy said this meant it was taking up to 45 days for shipments to clear Customs.

"It is a political retaliation against Australia because Australia made some unfriendly comments about China and because of the Huawei incident," one commodity analyst in China who did not want to be named said. "China is using coal as a weapon to fight back against Australia."


A second analyst also linked the move to politics, warning it would have a long-term impact on the industry because traders would no longer feel confident about ordering Australian coal in case there were further delays. However, the analyst also said China was seeking to boost domestic coal prices as the economy slowed and the delays should not be solely attributed to diplomatic tensions.

"I won't link the delay with any political motivation," Chen Hong, director of the Australian Studies Centre at Shanghai's East China Normal University said.

"China is not manipulating trade to exert political pressure. However, the diplomatic standstill has been doing on for two years and recurrent anti-China incidents have been counterproductive."

Professor Chen also said the Morrison government had been prudent by not jumping to conclusions on the cybersecurity attack, but he was critical of some media and think tanks which had labelled China as the "key suspect".

BHP's Mr Mackenzie said delays in approvals were affecting all companies importing coal, both metallurgical and thermal, into China.

"There are a number of things happening in China relating to how they plan and moderate, if you like, imports versus their own domestic production. And 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," he said.