President Trump on Thursday claimed, without citing evidence, that pharmaceutical companies were behind a push by House Democrats to impeach him.

"I wouldn't be surprised if the hoax didn't come a little bit from some of the people that we're taking on," Trump said at an event in Florida Thursday where he was discussing drug pricing.

House Democrats have launched an impeachment inquiry after Trump put pressure on Ukraine's president to announce an investigation into Joe Biden, a leading contender for the Democratic presidential nomination.

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President Donald Trump claimed without citing evidence that pharmaceuticals companies seeking to punish his administration for pushing to lower drug prices could secretly be behind an impeachment drive launched by House Democrats.

"Lowering the cost of prescription drugs, taking on the pharmaceutical companies, you think that's easy, it's not easy. It's not easy.

"I wouldn't be surprised if the hoax didn't come a little bit from some of the people that we're taking on," Trump said at an event in Florida Thursday, according to a pool report.

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"They're very powerful, they spend a lot of money, spend I think more money than any other group in the world actually in terms of lobbying and lobbying abilities.

"And I wouldn't be surprised if some of the nonsense that we all have to go through, but that I go through, wouldn't be surprised if it was from some of these industries like pharmaceuticals that we take on."

Democratic House Speaker Nancy Pelosi is a leading supporter of the impeachment inquiry. Getty Images/Mark Wilson

The president has long sought to discredit political rivals and burnish his populist credentials by spreading conspiracy theories alleging plots to unseat him.

Trump was speaking at the Florida event ahead of signing an executive order to expand Medicare, partly by increasing involvement from Medicare Advantage, that is run by private insurers.

He attended the event as Democrats pursuing an impeachment inquiry questioned former US diplomat Kurt Volker over efforts by the president to have Ukraine investigate domestic political rival Joe Biden.

The president also pushed Ukraine to back a groundless conspiracy theory alleging that Ukrainian officials hacked the 2016 presidential election.

Read more: Ukraine's president complained he didn't want to be used as a political 'instrument' by Trump over his push to investigate Joe Biden, US diplomatic text messages say

"Not to be so frank, but that is a ridiculous question," Holly Campbell, spokesperson for PhRMA — a trade group representing the pharmaceuticals industry — told Politico in response to a question about Trump's claim of a plot by drugs companies.

Trump has spoken in support of lowering drugs prices — an issue on which there is rare bipartisan agreement between Republicans and Democrats — but attempts by lawmakers and the White House to broker a bill have so far been unsuccessful.