US president Donald Trump was overheard asking about “the investigations” he wanted Ukraine to pursue that are central to the impeachment inquiry, a senior diplomat has said.

William Taylor, the top US diplomat in Ukraine, revealed the new information as the House Intelligence Committee yesterday opened extraordinary hearings on whether the 45th President of the United States should be removed from office.

Mr Taylor said his staff recently told him they overheard Mr Trump speaking on the phone to another diplomat, Ambassador Gordon Sondland, at a restaurant the day after Mr Trump’s July phone call with the new leader of Ukraine.

The staff explained that Mr Sondland had called the President and they could hear Mr Trump on the phone asking about “the investigations”, he said.

The ambassador told the President the Ukrainians were ready to move forward, Mr Taylor testified at the proceedings, which were broadcast live, and on social media, from a packed hearing room on Capitol Hill.

The inquiry was launched after an anonymous whistleblower’s complaint that Mr Trump, in the July phone call, pressured Ukrainian President Volodymyr Zelensky to investigate Democratic foe Joe Biden and Biden’s son – all while the US was holding up US military aid.

Mr Trump has pushed back vigorously, insisting he did nothing wrong in his dealings with Ukraine. “New hoax. Same swamp,” read one tweet from the White House that Mr Trump retweeted as proceedings began.

He also said he was “too busy” doing the people’s business to watch the impeachment hearings. “It’s a witch hunt, it’s a hoax, I’m too busy to watch it,” Mr Trump said.

At the start of proceedings, Adam Schiff, the Democratic chairman of the Intelligence Committee, outlined the question at the core of the impeachment inquiry – whether the president used his office to pressure Ukraine officials for personal political gain.

“The matter is as simple and as terrible as that,” said Mr Schiff. “Our answer to these questions will affect not only the future of this presidency but the future of the presidency itself, and what kind of conduct or misconduct the American people may come to expect from their commander in chief.”

Republicans immediately pushed Democrats to hear in closed session from the anonymous whistleblower. Mr Schiff denied the request at the time but said it would be considered later.

The top Republican on the panel, Devin Nunes, said Mr Trump “would have a perfectly good reason for wanting to find out what happened” if there were indications that Ukraine meddled in the 2016 presidential election. National security officials have told Congress they do not believe Ukraine meddled in the 2016 election.

Mr Nunes accused the Democratic majority of conducting a “scorched earth” effort to take down the president after the special counsel’s Russia investigation into the 2016 election failed to spark impeachment proceedings.

“We’re supposed to take these people at face value when they trot out new allegations?” said Mr Nunes, a top Trump ally. The Democrats plan to call nine additional witnesses in the next eight days.