The US chief justice has been sworn in to preside over President Donald Trump’s impeachment trial, before he immediately swore in the entire Senate body in turn with an oath to ensure “impartial justice” as jurors.

Chief Justice John Roberts made the short trip across the street from the Supreme Court in Washington DC before being ushered to the Senate chamber on Thursday.

The US Constitution mandates the chief justice serve as the presiding officer.

Justice Roberts, who has long insisted judges are not politicians, is expected to serve as a referee for the proceedings rather than an active participant. Senators will ultimately render the verdict.

Earlier on Thursday, House Democrats prosecuting the case stood before the Senate and formally read the articles of impeachment against Mr Trump.

House Intelligence Committee Chairman Adam Schiff read the two charges against U.S. President Donald Trump as Senate formally opened the #impeachment trial https://t.co/oEjZspklI2 pic.twitter.com/5uXM7vWECx

— Reuters (@Reuters) January 16, 2020

Senators filled the chamber, sitting silently at their seats under strict trial rules that prohibit talking or phones, as the ceremonial protocol shifted the proceedings out of Speaker Nancy Pelosi’s Democratic-run House to the Republican-majority Senate.

Seven lawmakers prosecuting the charges, led by Adam Schiff of the Intelligence Committee and Jerrold Nadler of the Judiciary Committee, made the solemn walk across the US Capitol for a second day.

“With the permission of the Senate, I will now read the articles of impeachment,” Mr Schiff said, standing at a lectern in the well of the chamber, a space usually reserved for senators.

“House Resolution 755 Impeaching Donald John Trump, president of the United States, for high crimes and misdemeanors,” he began, reading the nine pages.

The other House prosecutors stood in a row to his side.

Mr Trump faces a charge that he abused his presidential power by pressuring Ukraine to investigate Democratic rival Joe Biden, using military aid to the country as leverage.

I JUST GOT IMPEACHED FOR MAKING A PERFECT PHONE CALL!

— Donald J. Trump (@realDonaldTrump) January 16, 2020

Mr Trump was also charged with obstructing Congress’s ensuing probe.

Ahead of the proceedings, the Government Accountability office said on Thursday that the White House violated federal law in withholding the security assistance to Ukraine, which shares a border with hostile Russia.

The president calls the impeachment a “hoax”, even as new information emerges about his actions toward Ukraine that led to the charges against him.

Ms Pelosi said new allegations from an indicted associate of Mr Trump lawyer Rudy Giuliani, Lev Parnas, only reinforce the need for the Senate to consider further testimony about the president’s actions toward Ukraine.

Ms Pelosi noted that typically a special prosecutor would investigate but she doubted that would happen.

“This is an example of all of the president’s henchmen,” Ms Pelosi said, “and I hope that the senators do not become part of the president’s henchmen.”

-AAP