Sondland has previously said that he had no knowledge that things were to do with each other.

In a testimony in the House of Representatives published on Tuesday, Sondland said he at one time told one of Ukraine's President Volodymyr Zelensky's advisers that the frozen military aid would probably not be released until Ukraine made it clear that Joe Biden and his son's Ukraine coup would be investigated .

That was the "service" Donald Trump asked the Ukrainian president for in a lengthy phone call between the two this summer.

Chief of Staff is called to testify

- In the absence of any reasonable explanation for the withholding of aid, I assumed that the withheld aid had been linked to the proposed anti-corruption statement, Sondland said in the testimony, which is part of the Democrat-led investigation into a possible trial by President Donald Trump.

The statement mentioned is what Zelenskyj was asked to make public in order to clarify that Ukraine has started an investigation by Biden, which is one of the Democrats' top names for next year's presidential election.

The message came shortly after it became clear that the White House Chief of Staff Mick Mulvaney had been called to the House of Representatives to give testimony to that investigation into the national court process.

"Based on evidence collected by the National Investigation Commission and by the general reporting, we believe that you are in possession of essential first-hand information and knowledge relevant to the House of Representatives investigation," the chairmen of the Intelligence, Foreign Affairs and Supervisory Committee write in the notice, requests that Mulvaney show up to testify on Friday.

Not asked to cooperate

The big question being investigated is whether the US president committed a serious crime when he called Ukrainian President Volodymyr Zelenskyj last summer and asked him to investigate the Democrat and former Vice President Joe Biden's son - while the White House temporarily frozen a military aid package to Ukraine.

The White House has not yet responded to the call by Mulvaney, but the Trump government has previously urged state employees not to cooperate with the investigation - which Trump himself calls a "witch hunt".

Some have nevertheless testified when called for interrogation by the committees, while others refused.

On Monday, the intelligence committee released prints of interrogation with, among others, former US Ambassador to Ukraine Marie Yovanovitch. Then it emerged that she felt and still feels threatened by Trump and people in his staff.