Lt. Col. Alexander S. Vindman, the top Ukraine expert at the National Security Council, has testified that he was alarmed by attempts by U.S. President Donald Trump to pressure Ukraine into investigating former vice president Joe Biden, so he reported the matter through legal channels.

Vindman made the statement at Tuesday's impeachment hearing, where he said his military background made him understand that when someone senior "asks you to do something, even if it's polite and pleasant, it's not to be taken as a request. It's to be taken as an order."

Vindman is one of four key witnesses who were lined up to testify at the House Intelligence Committee House on Tuesday. The others are : Jennifer Williams, an adviser to Vice President Pence, Tim Morrison, another senior NSC official, and Kurt Volker, a former envoy to Ukraine.

Democrats are seeking to prove that President Trump leveraged military aid and a White House meeting in exchange for investigations of Joe Biden, his son Hunter and other Democrats.

Trump has denied any wrongdoing, labelling the impeachment inquiry a "witch hunt."

If the impeachment is successful, Donald Trump will become just the third president in U.S. history to be impeached.