The US military is about to complete a $60m expansion to its prison at the Bagram air base in Afghanistan, where it holds more than 600 so-called enemy combatants.
The near doubling of the prison size comes as human rights group Amnesty International urged Barack Obama, the US president, to give better legal protection to the detainees there, Aljazeera reported.
Obama has been widely praised for moving to shut down the US prison at Guantanamo Bay, Cuba, within days of taking office last month.
But with his move to send 17,000 more US troops to Afghanistan to shore up its operations there, the Bagram prison looks set to become more visible and controversial.
Rumi Nielson-Green, a spokeswoman for the US military, told Al Jazeera that the detainees held at Bagram were "unlawful enemy combatants".
"They are individuals who have been removed from the battlefield because they are dangerous to our forces or our coalition partners," she said.
Amnesty urged Obama to continue its break from his predecessor's "unlawful detention policies" by ensuring "all US detentions in Afghanistan comply with international law" and giving the detainees access to US courts to challenge their detentions.
"Judicial review is a basic safeguard against executive abuse and a protection against arbitrary and secret detention, torture and other ill-treatment and unlawful transfers from one country or government to another," Amnesty said.
"In the absence of judicial oversight, detainees in Bagram, as at Guantanamo, have been subjected to just such abuses - even children have not been spared."
The rights group says most of the 615 detainees being held at Bagram without access to courts or legal counsel are Afghan nationals, and some have been held for years.
While the Obama administration has committed itself to resolving the 240 or so Guantanamo cases within a year, it has not stated its intentions on Bagram.
John Bates, a US district court judge who is hearing petitions filed by four detainees held at Bagram for years, has given the administration until Friday to "refine" its position on the use of Bagram air base as a detention facility and whether it believes the detainees can challenge their detention in US courts.
The administration of George Bush, Obama's predecessor, had argued that as "enemy combatants", Bagram detainees had no right to challenge their detention in US courts - the same argument it made concerning detainees at Guantanamo.
But the US Supreme Court ruled last year that inmates at Guantanamo had the right to challenge their detentions in US courts, a ruling that rights groups hope will be extended to Bagram detainees.