Federal Treasurer Jim Chalmers has ordered another six companies to divest their holdings in Northern Minerals as concerns grow in Canberra that Chinese-backed investors are trying to seize control of the rare earths mining company.The six investors, Hong Kong Ying Tak Ltd, Real International Resources Ltd, Qogir Trading & Service Co Ltd, Chuanyou Cong, Vastness Investment Group Ltd and Zhongxiong Lin, hold around 17 per cent of the company, and will now have to sell their holdings within the next two weeks.A spokesman for Mr Chalmers did not lay out exactly why he had taken this step, simply saying the decision was "entirely consistent with advice from Treasury and the Foreign Investment Review Board" and was "about protecting our national interest and ensuring compliance with our foreign investment framework"."We operate a robust and non-discriminatory foreign investment framework, and will take further action if required to protect our national interest in relation to this matter," he said.Rare earths are a critical part of modern military technology and are used in precision guided weapons. (ABC Kimberley: Rebecca Nadge)The order comes in amid a long-running saga over control of the company, which is trying to develop its Browns Range Heavy Rare Earths Project in the East Kimberley with hopes of producing large quantities of dysprosium and terbium: critical elements for the magnets used for military, computing and clean energy technologies.Northern Minerals is also seen as a key player in efforts by both the US and Australia to break China's stranglehold on the critical minerals supply chain and is already on track for about $500 million in funding from the Export Import Bank of the United States.Browns Range aims to become the world's largest producer of dysprosium outside of China. (ABC Rural)One of the Chinese investors ordered to sell its shares, the Beijing-based Vastness Investment Group, tried to topple the company's chairman before abandoning its bid earlier this year.The government has also launched multiple interventions seemingly aimed at protecting the company from Chinese commercial entities stalking it.In 2023 Mr Chalmers blocked a Chinese-linked investment vehicle Yuxiao Fund from increasing its stake in the company, and the year after he ordered another five China-linked companies to divest from it.But the Foreign Investment Review Board wrote to Northern Minerals earlier this year, saying it believed three of the blocked investors had breached that order by transferring their shares to Hong Kong Ying Tak Ltd, one of the six companies ordered to divest today.Northern Minerals has also delayed its annual meeting as it tries to resolve the identity of its shareholders.The Browns Range pilot plant is located in the remote East Kimberley near the NT border. (ABC Kimberley: Rebecca Nadge)John Coyne from the Australian Strategic Policy Institute told the ABC that the government "appears to have concluded" that several China-based investors have "ignored repeated direction, and it has acted accordingly and appropriately"."It sends an important signal that Australia is far more willing to use investment policy as a tool of economic security," Dr Coyne said."Australia has been very clear for several years that critical minerals are no longer simply a commercial issue."They now sit at the centre of strategic competition, industrial resilience and economic security."Northern Minerals has entered a trading halt, and in a statement to the ASX said it was "currently considering" the treasurer's orders and "will make a further announcement once it has done so".The ABC has tried to contact the six investors ordered to divest their shares, but has so far been unable to reach any of them.