US Attorney General Merrick Garland on Thursday issued a moratorium on federal executions while a review of policy and procedures is pending to address "serious concerns" about the practice.

The move is a sharp break from the policies of former president Donald Trump's administration, which carried out a record number of executions.

However, no federal executions have occurred since President Joe Biden, who is known for his opposition to capital punishment, was inaugurated.

"Serious concerns have been raised about the continued use of the death penalty across the country," Garland wrote in a two-page statement announcing the moratorium. 

These include "arbitrariness in its application, disparate impact on people of colour, and the troubling number of exonerations in capital and other serious cases."

No federal executions will take place "while a review of the Justice Department's policies and procedures is pending," he wrote.

The Justice Department "must ensure that everyone in the federal criminal justice system is not only afforded the rights guaranteed by the Constitution and laws of the United States, but is also treated fairly and humanely," Garland said. 

"That obligation has special force in capital cases."

In the United States most executions are carried out by states, not the federal government.

Federal crimes that can carry the death penalty usually involve drug trafficking, terrorism or espionage. 

The US government had not carried out any execution in 17 years - but starting in July 2020 through the final days of the Trump administration, an unprecedented 13 federal prisoners were executed.