WASHINGTON - The White House has officially refused to co-operate with the impeachment inquiry against President Donald Trump.

A letter sent to Democratic leaders rejected it as "baseless" and "constitutionally invalid".

Three Democratic-led House committees are investigating Mr Trump.

The inquiry is trying to find out if the president held back aid to Ukraine to push it to investigate Joe Biden, who is running for the Democratic presidential nomination.

The White House letter comes hours after the Trump administration blocked the US ambassador to the European Union from appearing before a congressional impeachment investigation.

White House counsel Pat Cipollone addressed the eight-page letter to the leading Democrat and House Speaker, Nancy Pelosi, and the three chairmen of the Democratic committees.

He accused the leaders of setting up an inquiry that "violates fundamental fairness and constitutionally mandated due process".

The letter insisted that, because Democrats did not hold a vote on launching impeachment proceedings in the House, the inquiry was not "constitutionally invalid".

The letter also accused Democrats of trying to change the 2016 election result, and "deprive the American people of the President they have freely chosen".

"In order to fulfil his duties to the American people... President Trump and his Administration cannot participate in your partisan and unconstitutional inquiry under these circumstances."

In response, Ms Pelosi called the letter "manifestly wrong" and accused Mr Trump of trying to "normalise lawlessness".

"Mr President, you are not above the law. You will be held accountable."(FA)