The impeachment trial of US President Donald Trump has started with the charges against him read out in the Senate.

Seven House representatives, who have been appointed as impeachment prosecutors, walked across the Capitol for the second day - after delivering the charges to the Senate on Wednesday.

"Hear ye, hear ye, hear ye!" called out the Senate's sergeant-at-arms as he started the ceremonial protocol ahead of the charges being read out.

Lead prosecutor Adam Schiff, who also leads the intelligence committee, then read out the formal charges against the president from the well of the Senate usually reserved for senators.

US Chief Justice John Roberts, who will preside over the trial, later administered the oath to all 100 senators who will serve as jurors and swear to deliver "impartial justice".

Chief Justice: "Do you solemnly swear that in all things appertaining to the trial of the impeachment of Donald John Trump, president of the United States, now pending, you will do impartial justice according to the Constitution and laws, so help you God?"

Senators: "I do." pic.twitter.com/3WAornlLPR

— CSPAN (@cspan) January 16, 2020

Thursday's ceremony was the formal start of the impeachment trial, but the actual arguments will not start until next Tuesday.

Mr Trump is accused of abusing his office by asking the Ukrainian president to investigate the son of his political rival Joe Biden.

However, the president has dismissed the charges as a "hoax".

He suggested Republican senators should vote to dismiss the impeachment articles without hearing any arguments, but they have indicated that is not something they will do.

There is an ongoing debate over how and when witnesses will be called, with Mr Trump's administration saying they should not be called - while most in the House say they should.

In this image from video, presiding officer US Supreme Court Chief Justice John Roberts swears in members of the Senate for the impeachment trial against President Donald Trump at the US Capitol in Washington DC | Image: AP/Press Association Images

During Bill Clinton's impeachment trial no witnesses were called, however they had all given evidence before the trial, and as a House representative said on Wednesday: "Did we really want to discuss sex in the Senate?"

Republicans control the Senate, the US's second chamber, by 53-47 and it would take 51 votes to approve rules or call witnesses during the trial.

If four Republican senators joined the Democrats side, they could insist on new evidence.

Mr Trump's trial will be only the third in US presidential history - after Andrew Johnson and President Clinton - and it will be the second to be televised.

Meanwhile the White House broke the law by withholding aid approved for Ukraine by US Congress.

The report by the Government Accountability Office (GAO), a non-partisan federal agency, came as the impeachment trial of the US president formally opened in the Senate.

The freezing of the aid is at the centre of the case against Mr Trump.

The agency said that delaying Congress-approved aid for Ukraine for "policy reasons", rather than technical budgetary needs, violates the law.

"Faithful execution of the law does not permit the President to substitute his own policy priorities for those that Congress has enacted into law," it said.

"The president is not vested with the power to ignore or amend any such duly enacted law."