US President Donald Trump on Tuesday pardoned four former government contractors convicted in a 2007 massacre in Baghdad that left more than a dozen Iraqi civilians dead and caused an international uproar over the use of private security guards in a war zone.

Trump also granted pardons to two people linked to a probe into alleged collusion between his campaign and Russia along with a list of others as time ticks away on his remaining weeks in office.

The moves drew even further controversy and came as the outgoing Republican continues to refuse to concede defeat to Democrat Joe Biden in the November election.

They add to pardons already issued to political allies of Trump, due to leave office on January 20.

The White House said in a statement that Trump had granted full pardons to 15 people and commuted all or part of the sentences for five others.

A full pardon was given to George Papadopoulos, a former Trump campaign adviser who admitted lying to federal investigators about his contacts with Russians.

Papadopoulos was a member of Trump's foreign policy advisory panel when he ran for president in 2016.

He pleaded guilty in October 2017 to lying to the FBI about contacts with a professor who promised to connect him to senior Russian officials.

He cooperated with investigators led by Robert Mueller, the special prosecutor who conducted a two-year probe into alleged collusion between the Trump campaign and Russia.

Papadopoulos spent 12 days in jail after his guilty plea.

“Today's pardon helps correct the wrong that Mueller's team inflicted on so many people,” the White House statement said.

Another full pardon was granted to Alex van der Zwaan, a Dutch lawyer who was also convicted in connection with Mueller's probe.

Full pardons were also granted to the four Blackwater security guards convicted over the 2007 killing of Iraqis.

The four guards for the Blackwater security firm convicted over the 2007 shootings included Nicholas Slatten, who had been sentenced to life.

They were convicted of opening fire in Baghdad's crowded Nisur Square on September 16, 2007, in a bloody episode that caused an international scandal and heightened resentment of the American presence.

The shooting left at least 14 Iraqi civilians dead and 17 wounded while perpetuating the image of US security contractors run amok. The Blackwater guards said they acted in self-defence in response to insurgent fire.

The White House statement said the four men, former members of the military, “have a long history of service to the nation”.

Others included on the list were three former Republican members of Congress.

'If you lie'

The announcement drew quick condemnation from a number of Trump critics, including Democratic Congressman Adam Schiff, the head of the House Intelligence committee.

“If you lie to cover up for the president, you get a pardon. If you are a corrupt politician who endorsed Trump, you get a pardon. If you murder civilians while at war, you get a pardon,” Schiff said in a statement.

The American Civil Liberties Union (ACLU) was particularly scathing on the pardon of the Blackwater security contractors behind the Iraq shooting.

“President Trump has hit a disgraceful new low with the Blackwater pardons,” said Hina Shamsi, director of the ACLU's National Security Project.

“These military contractors were convicted for their role in killing 17 Iraqi civilians and their actions caused devastation in Iraq, shame and horror in the United States, and a worldwide scandal. President Trump insults the memory of the Iraqi victims and further degrades his office with this action.”

Outgoing Republican Congressman Will Hurd also slammed the move on Twitter, saying that “pardoning privileged people of elected office and power for committing crimes they knew they were breaking and have pleaded guilty to is not conservative at all.”

“I know nothing he does is surprising any more, but what an obscene, partisan, and gross abuse of power,” said Democratic Senator Chris Van Hollen on Twitter.

“This is the swamp at its muckiest. January 20th can't come soon enough,” he said.