New York: In explosive testimony to the impeachment inquiry into US President Donald Trump, the US ambassador to the European Union confirmed there was an explicit "quid pro quo" linking a White House visit to politically motivated investigations.

Gordon Sondland, a Trump appointee and donor, said the highest levels of the White House and State Department were involved in the push for Ukraine to announce investigations into the family of potential 2020 rival Joe Biden and into debunked conspiracy theories about the 2016 election.

This level of involvement included Secretary of State Mike Pompeo, Vice-President Mike Pence and former national security adviser John Bolton.

"They knew what we were doing and why," Sondland told the House Intelligence Committee on Wednesday, local time (Thursday AEDT). "Everyone was in the loop. It was no secret."