KABUL, Afghanistan — More than 3,000 detainees held by the U.S. military will be transferred to Afghan control within six months under an agreement signed Friday between the United States and Afghanistan.

While the United States will retain the power to veto any detainee's release, the prisoner agreement meets a key demand of Afghan President Hamid Karzai's government as the two sides try to hammer out the details of U.S. engagement in Afghanistan following the expected end of American combat operations by 2014.

The first batch of about 500 detainees is likely to be transferred within 45 days from the U.S.-run detention center at the Bagram military complex, north of Kabul.

The agreement would apply only to Afghan detainees, said a senior U.S. official involved in the negotiations, speaking on condition of anonymity because of the sensitivity of the talks. About 50 non-Afghans — primarily al Qaida suspects from Pakistan, Arab countries and elsewhere — will remain in U.S. custody at Bagram.

The U.S. will build 11 new units at Bagram to house the detainees, as well as nine units at Pul-e-Charki prison on the outskirts of Kabul.

"At the end of the six-month period, the Afghans will have (full legal) custody of the Afghan prisoners," the U.S. official said.

The Afghan government would conduct administrative reviews of the detainees' cases but would have to consult with U.S. authorities before releasing any of them, effectively giving the United States the ability to block a prisoner from going free. Detainees whose cases are disputed will only be freed with the joint approval of the commander of the U.S.-led coalition, Marine Gen. John R. Allen, and Afghan Defense Minister Abdul Rahim Wardak.

The agreement comes after protracted and at times tense negotiations between U.S. and Afghan officials. Recent media reports have suggested U.S. negotiators threatened to abandon the talks. While U.S. officials didn't publicly disclose details, it's widely believed that American negotiators were seeking assurances that detainees transferred to Afghan custody wouldn't be able to secure their release by bribing local officials.

While U.S. officials called it a breakthrough, serious obstacles continue to stand in the way of the long-term partnership that the United States is seeking to ensure a long-term military role in Afghanistan. They include demands by Karzai that U.S.-led NATO forces end the controversial "night raids" on suspected insurgent hideouts — which coalition officials say have eliminated thousands of Taliban leaders and operatives — and differences over what role U.S. special forces would play after the bulk of American troops withdraw.

Most of the prisoners held at Bagram are suspected Taliban insurgents, and the U.S. also had reportedly been concerned that some detainees might be freed by Karzai in an effort to advance negotiations with the Taliban on a settlement to the decade-long war.

Defending the time taken to negotiate the transfer agreement, the U.S. official said the challenge had been "to find a way to give Afghanistan the recognition of its sovereignty ... while also preserving and protecting the humane treatment standards of these detainees."

He said that U.S. forces would remain at the detention facility for a year in an "advise and support" role to ensure that Afghan forces "provide humane and secure detention for those Afghan detainees, in accordance with international standards."