US Ambassador the the European Union (EU) Gordon Sondland testified that he followed President Donald Trump's orders to put pressure on Ukraine to investigate his Democratic rival Joe Biden at the Senate Intelligence Committee hearing in Washington DC on Wednesday.
He claimed the orders came through Trump's personal lawyer Rudy Guiliani.
"Giuliani was expressing the desires of the president of the United States and we knew these investigations were important to the president," said Sondland.
Guiliani also specifically mentioned Burisma and the 2016 presidential election, according to his statement.
The inquiry is investigating whether Trump withheld military aid to Ukraine as a precondition.
If found guilty in a majority vote in the House of Representatives, Trump will face an impeachment trial in the Senate. A two-thirds majority would be required to remove Trump from office.