WASHINGTON, Dec 28 (Reuters) - U.S. forces are working to complete a major expansion of detention facilities in Iraq as a growing prisoner population nears 15,000, the military said on Wednesday, amid questions about when Iraq's government will be capable of taking control.

The U.S. military, in the process of training hundreds of Iraqis to work as security guards, said it had no timetable for turning over detention operations to the Iraqi government. The prisoner population, consisting of suspected insurgents, has more than doubled since autumn 2004.

"Iraqi forces must meet all standards of care and custody in full accordance with international law and respecting the human rights of all individuals being detained," said Lt. Col. Barry Johnson, a senior military spokesman in Baghdad.

Recent incidents have generated concern about Iraqi-run detention operations. During a raid at a Baghdad bunker, U.S. forces last month found 173 men and teenage boys held by Iraq's Interior Ministry, many malnourished, beaten and showing signs of torture.

On Wednesday, at least nine prisoners and guards were killed and a U.S. soldier was wounded in a gunbattle at an Iraqi-run Baghdad jail.

The United States itself faced international condemnation last year over revelations American forces had abused detainees at Abu Ghraib prison on the outskirts of Baghdad.

Lt. Aaron Henninger, another military spokesman, said U.S. forces held more than 14,600 detainees in Iraq, primarily at four sites. The military earlier in the year launched a $51 million expansion of detention facilities to accommodate a growing population. Work is due to be completed in April, Henninger said.

The biggest is Camp Bucca near Umm Qasr in the south, holding 8,011 detainees. Abu Ghraib, notorious also as a torture center under deposed President Saddam Hussein, holds 4,648.

Fort Suse, at a Russian-built former military barracks near Sulaimaniya in northern Iraq, holds 1,347. Camp Cropper near Baghdad airport, holds 124 detainees, including many "high-value" prisoners. A further 542 are held at smaller facilities.

CROWDED PRISONS

Existing facilities are strained, with the population at 123 percent of "optimal capacity," the military said. In the past five months, the number of detainees has grown by 3,800, Henninger said. At the same time, U.S. forces have added space for 3,170 detainees.

Detention operations have tied up a significant number of U.S. troops -- 3,745 -- at a time when the Pentagon is planning to reduce the overall force in 2006.

Amnesty International official Jumana Musa said it would be premature to turn over the detainees to the Iraqi government now, considering the revelations of mistreatment of detainees by Iraqi forces and the lack of a fully functional Iraqi justice system.

"The idea of turning over thousands of detainees to a system that's barely gotten itself together, much less has shown itself to be reliable, would be frightening," Musa said.

A four-step plan for turning over detention operations starts with training Iraqis in a classroom setting to be prison guards, then having them work side by side with U.S. personnel inside jails. Henninger said 300 Iraqis were working alongside U.S. guards at Fort Suse, while 150 were being trained at Fort Suse and 126 at Camp Bucca.

The final two steps have not begun: trained Iraqis conducting detention operations with U.S. transition teams present, and wrapping up that oversight when the Iraqis are ready to take complete control.

"The Iraqis are committed to doing this right and will not rush to failure. The transition will be based on meeting standards, not on a timeline," Johnson said.