The US military has transferred a female detainee, captured during a raid on the Islamic State, over to Iraqi Kurdish custody after nearly three months of holding her in low-profile captivity for interrogation.

The Pentagon had remained silent about its plans for Umm Sayyaf since a raid on 15 May in eastern Syria that led to her capture. Sayyaf, who is reportedly an Iraqi national, was the first detainee held by the US in its year-long war against Isis.

But on Thursday, the Pentagon announced that the woman, whose real name is Nasrin As’ad Ibrahim, is now being held by the interior ministry of the semi-autonomous Kurdistan Regional Government.

A Pentagon statement nevertheless referred to Umm Sayyaf as being in the “custody of the government of Iraq,” even though the two entities have parallel security structures.

It was unclear why the US turned the woman over to Iraqi Kurdish custody instead of the custody of the Iraqi government. Pentagon officials did not immediately respond to requests for an explanation of the decision.

Numerous reports have stated that the US military has been interrogating Sayyaf. Yet in its statement on Thursday, the Pentagon still referred to her as only a “suspected” member of Isis, raising questions about how fruitful or valuable such interrogation has proven.

In a statement late on Thursday, the Pentagon said without elaboration that the Obama administration had determined the transfer “would be appropriate with respect to legal, diplomatic, intelligence, security, and law enforcement considerations”.

Human rights advocates have questioned the propriety of transferring Sayyaf back to Iraq, citing the Iraqi government’s “widespread abuse of prisoners,” as Human Rights Watch’s Andrea Prasow told the Guardian in May. The group has also documented the abuse of Isis prisoners in Kurdish Peshmerga hands, the militant force of Iraqi Kurdistan at the front lines of the battle against the jihadist Isis forces.

The Pentagon had remained silent about what it would ultimately do with Sayyaf since US special operations forces raided an Isis location in eastern Syria – killing her husband, said to be a mid-ranking Isis member – almost three months ago. The Obama administration has taken relatively few militants into custody outside Afghanistan, and those it has captured beyond that country have most often been transferred to US federal courts for prosecution.

After the capture, US defense secretary Ashton Carter said the US suspected Sayaff “played an important role in Isil’s terrorist activities, and may have been complicit in what appears to have been the enslavement of a young Yezidi woman”. No evidence in support of that supposition has been provided.

The Pentagon said transferring Sayaff was “consistent with DoD [Department of Defense] policy to detain, interrogate, and, where appropriate, seek the prosecution of individuals who are captured on the battlefield”.

The Guardian