President Donald Trump. File picture: AP Photo/Charlie Riedel.

Washington - US President Donald Trump has ramped up his rhetoric on the impeachment inquiry from Democratic lawmakers, taking to Twitter to label the investigation a "coup" designed to "take away the power of the people."

"As I learn more and more each day, I am coming to the conclusion that what is taking place is not an impeachment, it is a COUP," Trump tweeted late Tuesday.

Last week, Democrats in US House of Representatives launched an impeachment inquiry that centres around a July 25 phone call that Trump held with Ukrainian President Volodymyr Zelensky.

In the call, Trump asked Zelensky to potentially open an investigation into the Republican president's political rival, ex vice president Joe Biden.

Trump has been using increasingly strong language to lash out at the Democrats and the inquiry, repeatedly referring to it as "a witch hunt."

Over the weekend, he tweeted that if he were to be removed from office through a full impeachment process, that could lead to "civil war," a remark that drew concern even from members of his own Republican Party.

On Monday, Trump dubbed the whistleblower whose complaint sparked the impeachment inquiry a "fake," and also questioned if Democrat Adam Schiff, the head of the House Intelligence Committee, should be arrested for treason.