Chinese investors have filed a lawsuit to international arbitration claiming $3.5 billion in damages from Ukraine over the sale of aircraft engine maker Motor Sich, local media said Monday. Chinese investors hold a 75 percent stake of Motor Sich, a large producer of aircraft and helicopter engines that is based in central Ukraine. Shares of Motor Sich have been frozen since April 2018 following the launch of a criminal probe into the possible illegal sale of the company to foreigners. Attempts by Chinese investors to unfreeze the shares in Ukrainian courts have been unsuccessful. "On December 5, 2020, Chinese investors of PJSC Motor Sich sent an appeal to international arbitration," the privately-owned group DCH -- a partner of the Chinese in the deal -- said in a statement quoted by the Interfax Ukraine news agency. The United States has expressed its concern over the acquisition and sought to block it. Former White House national security adviser John Bolton said in an August 2019 visit to Kiev that Washington was worried "about the loss of military technology to China". Motor Sich cut ties with Russia, its biggest client, after the annexation of Crimea in 2014 and has since struggled to find new markets. ant-dg/acl/erc