The United States Congress opened the impeachment trial of President Donald Trump on Thursday, with House Democrats reading the formal charges from the well of the US Senate before the swearing-in of all 100 senators as jurors for only the third impeachment trial in US history.

"Hear ye, hear ye, hear ye!" said the Senate's sergeant at arms, calling the proceedings to order.

Seven legislators prosecuting the charges, led by Representative Adam Schiff of the Intelligence Committee and Representative Jerrold Nadler of the Judiciary Committee, made the solemn walk across the Capitol for a second day. It's the start of ceremonial protocol that shifts the proceedings out of Speaker Nancy Pelosi's Democratic-run House to the Republican-majority Senate.

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Schiff, standing in the well of the Senate usually reserved for senators, began reading the resolution, "Impeaching Donald John Trump, president of the United States for high crimes and misdemeanors."

Later in the day, Chief Justice John Roberts administered the jurors' oath to senators who swore to deliver "impartial justice".

The events, unfolding during an election year as Trump seeks another term, will be a test not only of his presidency but also of the nation's three branches of power and its system of checks and balances. Several senators are running for the Democratic Party's nomination to challenge Trump in November.

New evidence emerges

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

Pelosi said new allegations from an indicted associate of Trump lawyer Rudy Giuliani, Lev Parnas, only reinforces the need for the Senate to consider further testimony about the president's actions towards Ukraine.

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," Pelosi said, "and I hope that the senators do not become part of the president's henchmen."

McConnell speaks as the impeachment trial against President Donald Trump begins in the Senate at the US Capitol [Senate Television/AP Photo]

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. Trump was also charged with obstructing Congress's ensuing probe. In advance of the proceedings, the Government Accountability Office said Thursday that the White House violated federal law in withholding the security assistance to Ukraine, which shares a border with hostile Russia.

Senate Majority Leader Mitch McConnell opened the chamber Thursday decrying Pelosi's decision to hand out "souvenir pens" after she signed the resolution to transmit the charges to the Senate.

"This final display neatly distilled the House's entire partisan process into one perfect visual," McConnell said. "It was a transparently partisan process from beginning to end."

Witnesses?

Democratic Senate leader Chuck Schumer renewed his party's request that the trial include new witnesses and documents not available for the House impeachment proceedings.

"What is the president hiding? What is he afraid of?'' Schumer said.

"The gravity of these charges is self-evident," he said. "The House of Representatives have accused the president of trying to shake down a foreign leader for personal gain."

The president has suggested recently that he would be open to a quick vote to simply dismiss the charges, but sufficient Republican support is lacking for that. Still, an eventual vote to acquit Trump is considered highly likely.

On Wednesday, in a dramatic procession across the US Capitol, House Democrats carried the charges to the Senate.

"Today, we will make history,'' Pelosi said as she signed the documents, using multiple pens to hand out and mark the moment. "This president will be held accountable."

House Intelligence Committee Chairman Adam Schiff and House Judiciary Committee Chairman Jerrold Nadler and other House impeachment managers, walk to the Senate chamber on Capitol Hill [Matt Rourke/AP Photo]

Moments later the prosecutors walked solemnly through the stately hall, filing into the Senate back row as the clerk of the House announced the arrival: "The House has passed House Resolution 798, a resolution appointing and authorising managers of the impeachment trial of Donald John Trump, president of United States."

Opening arguments are to begin next week after the Martin Luther King Jr holiday on Monday.

Earlier on Wednesday, the House voted 228-193, almost entirely along party lines, ending a weeks-long delay to deliver the charges with a tally reflecting the nation's split.

The top Republican in the House, Kevin McCarthy of California, said Americans will look back on this "sad saga" that tried to remove the president from office with the "weakest case".

The president's team expects acquittal with a Senate trial lasting no more than two weeks, according to senior administration officials. That would be far shorter than the trial of President Bill Clinton, in 1999, or the first one, of President Andrew Johnson, in 1868. Both were acquitted.

The seven-member prosecution team is led by the chairmen of the House impeachment proceedings, Representative Adam Schiff of the Intelligence Committee and Jerrold Nadler of the Judiciary Committee, two of Pelosi's top lieutenants.

On Wednesday, Schiff released new records from Lev Parnas, an associate of Trump lawyer Rudy Giuliani, about the Ukraine strategy, including an exchange with another man about surveilling later-fired US Ambassador Marie Yovanovitch.

Schiff said the new evidence should bring more pressure on McConnell, who is reluctant to allow witnesses to testify and prefers swift acquittal. The White House has instructed officials not to comply with House subpoenas for testimony and documents.

"The challenge is to get a fair trial," Schiff said in an interview with The Associated Press. "It shouldn't be a challenge - if the senators are really going to live up to their oath to be impartial, they'll want a fair trial. That's obviously not where Mitch McConnell is coming from."

House managers

The managers are a diverse group with legal, law enforcement and military experience, including Hakeem Jeffries of New York, Sylvia Garcia of Texas, Val Demings of Florida, Jason Crow of Colorado and Zoe Lofgren of California.

Two are freshman legislators - Crow - a former Army Ranger who served in Iraq and Afghanistan, Garcia - a former judge in Houston. Demings is the former police chief of Orlando and Jeffries is a lawyer and member of party leadership. Lofgren has the rare credential of having worked on the congressional staff investigation of President Richard Nixon's impeachment - he resigned before the full House voted on the charges - and then being an elected legislator during Clinton's.

Republican Senator Susan Collins of Maine is leading an effort among some Republicans, including Mitt Romney of Utah, Lisa Murkowski of Alaska and Lamar Alexander of Tennessee, to consider Senate witnesses. She told reporters she was satisfied the rules will allow votes on that.

Romney said he wants to hear from John Bolton, the former national security adviser at the White House, who others have said raised alarms about the alternative foreign policy towards Ukraine being run by Giuliani.

Any four senators could force an outcome. Republicans control the chamber, 53-47, but it takes just 51 votes during the trial to approve rules or call witnesses. It also would take only 51 senators to vote to dismiss the charges against Trump.