US President Donald Trump has issued a number of pardons during his time in the White House and is expected to deliver more before he leaves on January 20, 2021.

Here is a look at the pardon powers of the US president and some notable and controversial pardons over the years:

The president's pardon power

The US Constitution gives a president the "power to grant reprieves and pardons for offences against the United States, except in cases of impeachment."

This allows the president to issue pardons or commute sentences for persons convicted of federal offenses but not for state crimes.

Notable historical pardons

The first presidential pardons were issued by George Washington in 1795. Washington pardoned two men who had been convicted of taking part in the Whiskey Rebellion, a protest against a tax on spirits.

Following the 1861-1865 Civil War between the North and the South, president Andrew Johnson issued an unconditional pardon to soldiers who had served in the Confederate Army.

President Jimmy Carter pardoned the more than 200,000 Vietnam War draft evaders in 1977.

Controversial pardons

President Gerald Ford issued a pre-emptive pardon in 1974 to disgraced former president Richard Nixon for any crimes Nixon may have committed while in the White House.

President Bill Clinton pardoned his half-brother, Roger Clinton, who had been convicted of cocaine possession, and Marc Rich, a billionaire and major donor to the Democratic Party who had been convicted of tax evasion and was a fugitive.

President George H.W. Bush pardoned former defense secretary Caspar Weinberger, former National Security Advisor Robert McFarlane and four other people involved in the Iran-Contra scandal.

President Barack Obama commuted the 35-year sentence of Chelsea Manning, a US Army private who had been convicted of providing classified material to WikiLeaks.

Trump and pardons

Trump has doled out pardons to political allies including campaign consultant Roger Stone and retired general Michael Flynn, his one-time National Security Advisor who pleaded guilty to making false statements to the FBI.

And on Tuesday he granted pardons to 15 more people and commuted all or part of the sentences for five others.

Among them are a full pardon for George Papadopoulos, a former Trump campaign adviser who admitted lying to federal investigators about his contacts with Russians.

Trump also granted a full pardon to Alex van der Zwaan, a Dutch lawyer who was also convicted in connection with the two-year probe into alleged collusion between the Trump campaign and Russia led by special prosecutor Robert Mueller.

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

And he granted full pardons to four Blackwater security guards convicted over the 2007 killing of at least 14 Iraqi civilians in Baghdad, including Nicholas Slatten, who had been sentenced to life.

The Justice Department has been investigating since August whether bribes were offered to White House officials to secure a pardon or commutation of sentence for an unnamed individual.

No charges have been filed.

• More to come? -

According to a New York Times report, Trump has also discussed with advisors the possibility of issuing pre-emptive pardons to his children, son-in-law Jared Kushner and his lawyer, Rudy Giuliani, before leaving office.

Trump has also reportedly asked lawyers about the possibility of issuing a pardon for himself for any crimes he might be charged with related to his time in office.