The United States has formally handed over control of a controversial prison to the Afghan government, despite a disagreement about the fate of some of its inmates.

A small ceremony at Bagram prison outside Kabul marked the handover of more than 3,000 prisoners from U.S. to Afghan authorities. Afghan President Hamid Karzai hailed the transfer of the prison and the inmates as a victory for Afghanistan's sovereignty.

But about 600 detainees remain in U.S. custody. They were captured following the March agreement that outlined the transfer of authority to the Afghans. Around 50 foreign detainees, mostly from Pakistan, also are not covered by the agreement and have not been put under Afghan oversight. Afghan officials insist that all detainees must be handed over within 72 hours.

Also, U.S. officials say they have suspended additional transfers because of concerns about the intentions of the Afghan government. It is not immediately clear what those concerns are, but they seem to have emerged after a meeting Saturday between President Karzai and General John Allen, the top commander of American and NATO forces in Afghanistan.

Rachel Reid with the human-rights group Open Society Afghanistan says the Afghan government fears a “mini-Guantanamo” might result if the Americans get to hold third-country nationals indefinitely in Afghanistan.

But a U.S. official tells VOA that those so-called third-country nationals are not covered in the March agreement.

Meanwhile in northern Afghanistan, a suicide bomber blew himself up in a crowded main square, killing 16 people including police officers.

Local officials say the attack occurred in the urban capital of Kunduz province. The city's police chief tells VOA that the blast wounded at least 20 people, including two women.

No one immediately took responsibility for the blast.