SYDNEY (Reuters) -Australia is very concerned about U.S. President Donald Trump's latest threat to raise tariffs on pharmaceutical imports by up to 250% over the longer term, its health minister said on Thursday.

Trump said this week that the United States would first impose a "small" tariff on foreign-made drugs before hiking it to 150% within 18 months and eventually to 250% in an effort to boost domestic production.

Mark Butler said the plan would be a blow to producers such as CSL, Australia's largest biotech company.

"And that is why we are working so hard to press the case for the continuation of free trade," he told reporters.

Medicines and pharmaceuticals are among Australia's top exports to the U.S, with around A$2.1 billion ($1.4 billion) in shipments last year, according to the Australian Bureau of Statistics.

There has been some bilateral tension over drug pricing in Australia.

In a January submission to the U.S. government, the Pharmaceutical Research and Manufacturers of America trade group said Australia's Pharmaceutical Benefits Scheme was "egregious and discriminatory" and "undervalued American innovation," putting jobs and exports at risk.

Butler said the national programme, which subsidises a wide range of prescription medicines for Australians, would not be changed to appease American companies and escape the threatened tariffs.

"To the extent that this is motivated by big pharma companies seeking to lobby the U.S. administration to water down the protections of our PBS, it's simply not up for negotiation under our government," he said.

($1 = 1.5302 Australian dollars)