Washington: The Senate impeachment trial on whether to remove US President Donald Trump from office formally began on Thursday even as a congressional watchdog found that the White House broke the law by withholding security aid for Ukraine approved by Congress.

The assessment from the nonpartisan Government Accountability Office was a setback for Trump, but it was unclear if it would figure in his trial in the Republican-led Senate given that key questions such as whether witnesses will testify or new evidence will be considered remain unanswered.

Democrat Adam Schiff, who heads a team of seven House of Representatives members who will serve as prosecutors, appeared on the Senate floor to read the two charges passed by the House on December 18 that accused Trump of abuse of power and obstruction of Congress arising from his dealings with Ukraine.

US Chief Justice John Roberts, wearing his black judicial robe, took an oath to preside over the trial and then swore in the assembled senators who will serve as jurors, asking if they would "do impartial justice."