US Ambassador to the EU Gordon Sondland, a key witness in the impeachment investigation of President Donald Trump AFP/SAUL LOEB

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 Sep 1 discussion with Andriy Yermak, a top advisor to Ukrainian President Volodymyr Zelensky.

He said it was the culmination of months of pressure placed on Kiev, primarily via Trump's personal lawyer Rudy Giuliani, to open "anti-corruption" investigations into the Bidens and into Trump's unfounded belief that Ukraine assisted the Democrats in the 2016 election.

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 US$1 million to his inauguration, was fully aware of the linkage made between the US$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."

The House of Representatives's impeachment investigation is focused on a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelenskyy to investigate former Vice President Joe Biden, a leading Democratic rival, and his son Hunter, who was on the board of a Ukrainian gas company that had been investigated for corruption. There has been no evidence of wrongdoing by the Bidens.

Trump froze nearly $400m in US military assistance to Ukraine shortly before speaking to Zelenskyy, prompting accusations from Democrats that he had misused taxpayer dollars destined for a vulnerable US ally for personal gain.

Former special representative for Ukraine negotiations testimony released

Congressional Democrats also released testimony from Kurt Volker, Trump's former special representative for Ukraine negotiations.

Witnesses have testified that Volker and Sondland, with Trump's secretary of energy, Rick Perry, were known as the "three amigos", responsible for Trump's unofficial channel to Ukrainian government officials.

Volker resigned as special representative in September. He testified to the House Foreign Affairs, Intelligence and Oversight panels for more than eight hours behind closed doors on October 3.

Sondland, a major Trump donor, testified on October 17.

Perry, a former Texas governor who said he was resigning from his Cabinet post as of December 1, has refused to testify so far.

Trump has denied wrongdoing, stating there was no "quid pro quo" (Latin for a "favour for a favour"). He has accused Democrats of unfairly targeting him in hope of reversing his surprise victory in the 2016 presidential election. So far, he has maintained strong support from fellow Republicans in Congress.

Most of those who have testified before the House panel are from the ranks of the State Department, including recalled US Ambassador to Ukraine Marie Yovonavitch, whose testimony was released Monday. Diplomats have testified to the mounting concerns in the State Department over Trump's interest in having a foreign ally investigate Biden.

Volker and Sondland testified they were disappointed after briefing Trump at the White House upon their return from Zelenskyy's inauguration in May as a new leader of the young democracy vowing to fight corruption.

That pivotal May 23 meeting raised red flags when Trump told them to work with Rudy Giuliani, his personal lawyer, on Ukraine issues./.

VNF/AFP/Reuters