In a statement, the ACLU said the Pentagon had promised to post 198 of these images online by Friday.

The Pentagon has pledged to release part of a trove of photographs related to the abuse of prisoners at US detention centers in Iraq and Afghanistan, a top US rights group said Wednesday.

The move would be the latest step in a legal saga between the US military and the American Civil Liberties Union, which filed a freedom of information lawsuit in 2004 seeking the release of some 2,000 photographs showing detainee mistreatment.

In a statement, the ACLU said the Pentagon had promised to post 198 of these images online by Friday.

Though the administration of President Barack Obama had said it would release the photos back in 2009, Congress passed an exemption to the Freedom of Information Act allowing for the images to be withheld if the defense secretary deemed them to threaten national security.

The US government opposes publishing the images because it fears they could provoke a violent backlash and place US forces and personnel overseas at heightened risk of attack.

In March 2015, a US judge ordered the government to release the photos, but the Pentagon appealed that ruling.

However, in November last year, Pentagon chief Ashton Carter declined to recertify 198 images as posing a risk to national security. Those are the pictures due to be released.

The ACLU said it would continue to seek the release of the remaining 1,800 or so images.

"We're still pressing our case for the remaining secret photos," the group said.

US soldiers were implicated in the torture and sexual humiliation of Iraqi prisoners at Baghdad's Abu Ghraib prison when the US military ran it in 2004, a scandal that first broke when photos showing soldiers abusing detainees were published in US media.

Between 2004 and 2006, 11 soldiers - including Lynndie England, who was seen smiling beside naked prisoners being subjected to sexual abuse - were convicted in court martials.