A top ally of President Donald Trump admitted he told a Ukraine official that military aid was contingent on Kiev investigating Trump's Democratic rival Joe Biden, testimony released Tuesday showed.

In some of the most damning evidence to the House impeachment inquiry of Trump yet, Gordon Sondland, the US ambassador to the European Union, said he told a senior Ukraine official that US military aid would likely not be released until Kiev made clear it would investigate Biden and his son's ties to Ukraine energy firm Burisma.

Sondland admitted he laid out the allegedly illegal quid pro quo, which Trump has repeatedly denied, in a September 1 discussion with Andriy Yermak, a top advisor to Ukrainian President Volodymyr Zelensky.

Sondland told Yermak "that resumption of US aid would not likely occur until Ukraine provided the anti-corruption statement that we had been discussing for weeks," he told investigators.

Sondland, appointed ambassador by Trump after donating $1 million to his inauguration, was fully aware of the linkage made between the $391 million in US military aid and Ukraine helping Trump prepare to fight for reelection next year.

"I presumed that the aid suspension had become linked to the proposed anti-corruption statement" Zelensky was told to make regarding Biden and 2016 probes.

Sondland admitted to impeachment investigators that he understood the linkage was "improper."

Asked if it was illegal, Sondland responded: "I'm not a lawyer, but I assume so."