More than a million coronavirus test kits destined for the US are sitting in a warehouse in China due to new restrictions imposed by the Chinese government on exports.

The test kits are stranded along with masses of face masks and other medical equipment made by US firms after they were unable to get the necessary clearances demanded by the new rules, according to the Wall Street Journal, which cited businesses and US diplomatic memos.

The move follows a decision by Donald Trump to force US companies to redirect surgical masks manufactured abroad back to America for use in the battle against the coronavirus. Canadian prime minister Justin Trudeau was among a number of foreign leaders who warned the move could prompt retaliatory measures against the US which would hamper its ability to effectively fight the outbreak.

China is a global hub for the manufacture of medical equipment around the world. A slowdown in supply there could have grave implications for the international community's ability to fight the pandemic.

Chinese officials have said the new export policies were aimed at ensuring essential medical supplies that the country needs were not exported from China, and that the goods leaving the country were of sufficient quality, according to the report. Extra checks and certification are now required before medical supplies can leave the country.

Some 1.4 million test kits for Covid-19 made by PerkinElmer, a healthcare equipment manufacturer based in Massachusetts, are currently being held in its Suzhou factory because the company does not yet have the approval required by the new rules, the Wall Street Journal reported, citing a State Department memo.

The same memo said that one local mayor in China had told 3M, a Minnesota-based firm that produces protective and healthcare equipment, that it relied on 3M’s N95 respirators for its own coronavirus prevention efforts and did not have an alternative supply.

While some of the supplies were eventually released, the new rules are causing a bottleneck that is delaying essential medical equipment at a time the US is facing severe shortages, according to the State Department.

Among the products delayed was a shipment of circuit boards used in ventilators manufactured by General Electric, according to the Wall Street Journal.

The new restrictions come at a critical stage in America’s battle against the coronavirus. The country has seen more than 670,000 confirmed cases of the coronavirus and 33,000 deaths — the highest in the world.

Medical professionals working on the frontline of the crisis have reported severe shortages of protective equipment. Many are forced to reuse protective masks due to short supply.

Earlier this month, Mr Trump invoked the Defence Production Act, a Korean War-era law, to compel 3M to sell equipment destined for Canada to the US government.

Mr Trudeau, Canada’s prime minister, issued a veiled warning in which he warned that medical equipment “goes both ways across the border,” and noted that thousands of nurses travel from Canada to work in Detroit every day.