President Donald Trump has reiterated that he did not place a "quid pro quo" over US military aid to Ukraine in order to secure a investigation of a political rival, following testimony from the US ambassador to the European Union that alleged the President did just that.

Key points: Mr Sondland said Mr Trump forced US diplomats to work with his personal lawyer Mr Giuliani

Mr Sondland said Mr Trump forced US diplomats to work with his personal lawyer Mr Giuliani The ambassador said he "never received a clear answer" on why the US suspended security aid to Ukraine

The ambassador said he "never received a clear answer" on why the US suspended security aid to Ukraine Mr Sondland said "Giuliani's requests were a quid pro quo for arranging a White House visit for President Zelenskiy"

Gordon Sondland — whose appearance in an impeachment hearing before Congress was watched especially closely as he was a Trump ally — said he believed the President was pressing Ukraine to investigate his potential 2020 rival Joe Biden.

"We followed the President's orders," Mr Sondland said in his prepared testimony to an open hearing of the House Intelligence Committee.

But Mr Trump later insisted he did nothing wrong in his dealings with Ukraine, casting the impeachment inquiry as a politically motivated effort to push him from office.

President Donald Trump has consistently denied that he placed strings on US military aid to Ukraine. ( AP: Patrick Semansky )

Speaking to reporters outside the White House, he said he wanted "nothing" from the Ukrainians and did not seek a quid pro quo.

He also distanced himself from Mr Sondland, who was major donor to his inauguration.

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'Everyone was in the loop'

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 35 seconds 35 s Mr Sondland said he "never received a clear answer" on why the United States suspended security aid to Ukraine.

In a blockbuster morning of testimony, the ambassador's opening remarks included several key details: He confirmed that he spoke with Mr Trump on a mobile phone from a busy Kiev restaurant the day after the President prodded Ukraine's leader to investigate Mr Biden.

He also said he kept Secretary of State Mike Pompeo and other top administration officials aware of his dealings with Ukraine on the investigations Mr Trump sought.

US ambassador to the European Union Gordon Sondland said he "followed the President's orders". ( AP: Pablo Martinez Monsivais )

Mr Sondland said he specifically told Vice-President Mike Pence he "had concerns" that US military aid to Ukraine "had become tied" to the investigations.

"Everyone was in the loop," Mr Sondland said.

"It was no secret".

He said Mr Trump forced US diplomats to work with his personal lawyer, former New York mayor Rudy Giuliani.

"We did not want to work with Mr Giuliani. Simply put, we played the hand we were dealt," he said.

"Mr Giuliani was expressing the desires of the President of the United States, and we knew that these investigations were important to the President."

Mr Sondland said Mr Trump held off on offering a summit with Ukraine's new President, Volodymyr Zelenskiy, as Mr Giuliani demanded that Kiev publicly announce that it was investigating Burisma, a gas company on which former vice-president Mr Biden's son Hunter held a paid board position.

Impeachment testimony has made former New York mayor Rudy Giuliani a central figure. ( AP: Charles Krupa )

Mr Giuliani also wanted Mr Zelenskiy to investigate a widely discredited conspiracy theory in which Ukraine planted evidence on a server of Mr Biden's Democratic Party to show that Russia interfered in the 2016 election.

"Giuliani's requests were a quid pro quo for arranging a White House visit for President Zelenskiy," Mr Sondland said.

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Mr Sondland also said US Secretary of State Mike Pompeo was aware and "fully supportive" of their efforts on Ukraine, providing a fuller role of the top US diplomat's role in the affair.

The ambassador said he "never received a clear answer" on why the United States suspended security aid to Ukraine, which is battling Russian-backed separatists, but that he "came to believe" it was also tied to the investigations sought by Mr Trump.

"I was adamantly opposed to any suspension of aid, as the Ukrainians needed those funds to fight against Russian aggression," he said.

"In the absence of any credible explanation for the suspension of aid, I later came to believe that the resumption of security aid would not occur until there was a public statement from Ukraine committing to the investigations of the 2016 election and Burisma, as Mr Giuliani had demanded," he said.

'Their own peril'

Mr Sondland was appearing in the second week of televised impeachment hearings, in which Democrats are seeking to establish whether Mr Trump abused the power of his office by leveraging military aid and a White House meeting to extract a commitment from Mr Zelenskiy to probe the Bidens.

US Ambassador to the European Union Gordon Sondland testifies before the US House Intelligence Committee. ( REUTERS: Jonathan Ernst )

Ahead of his request that Mr Zelenskiy carry out the two investigations, Mr Trump froze $391 million in US security aid approved by Congress to help Ukraine combat Russia-backed separatists in the eastern part of the country.

Democrats have accused Mr Trump of using the frozen aid and Mr Zelenskiy's desire for an Oval Office meeting as leverage to pressure a vulnerable US ally to dig up dirt on political adversaries.

"I think we know now … that the knowledge of this scheme was far and wide and included, among others, Secretary of State Pompeo as well as the Vice-President," Adam Schiff, the Democratic chairman of the Intelligence Committee said, referring to Mr Pence.

Mr Schiff said Mr Pompeo and Mr Trump "have made such a concerted and across-the-board effort to obstruct this investigation and this impeachment inquiry. They do at their own peril."

The House investigation could conceivably wrap up this week, with evidence then sent to the House Judiciary Committee to draw up articles of impeachment.

Mr Trump's impeachment by the Democratic-controlled House would place Mr Trump on trial in the Senate, where a Republican majority could protect him from removal.

AFP/Reuters