An Iraqi security force under Prime Minister Nuri Kamal al-Maliki’s direct command held hundreds of detainees from northern Iraq in an undisclosed prison in Baghdad, torturing dozens of them, until the country’s human rights minister and the United States intervened late last month, Iraqi and American officials said.

Mr. Maliki ordered the prison closed and said he had been unaware it existed, according to the officials. His move brought the release of 71 detainees and the transfer of others to established prisons, but more than 200 remain in the place, on the grounds of the Old Muthanna military airfield, in northern Baghdad. All of the detainees were apparently Sunni Muslims.

American diplomats visited the prison on Wednesday, the officials said, and pressed Mr. Maliki’s government to investigate the circumstances of its creation and the treatment of detainees there, originally 431 in all.

In an interview, the minister of human rights, Wijdan Salim, praised Mr. Maliki for moving to close the prison and to order an investigation of what happened inside.

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