By Barbara Starr | CNN | Aug. 7, 2008

WASHINGTON (CNN) -- The U.S. military is segregating violent Iraqi prisoners in wooden crates that in some cases are not much bigger than the prisoners.

The military released photos of what it calls the "segregation boxes" used in Iraq.

Three grainy black-and-white photos show the rudimentary structures of wood and mesh. Some of the boxes are as small as 3 feet by 3 feet by 6 feet tall, according to military officials, although they did not release a picture of a box that size.

The average Iraqi male is 5 feet 9 inches tall, according to the Iraqi Ministry of Health. That leaves little room for a prisoner to move once put in the detention box.

The photos were made public after a blogger filed a Freedom of Information Act request in 2005.

The military said the boxes are humane and are checked every 15 minutes. It said detainees, who stand in the boxes, are isolated for no more than 12 hours at a time.

"Someone in a segregation box is actually observed more than those anywhere else," said Maj. Neal Fisher, a spokesman for Task Force 134, the Marine unit in charge of detainees. "Their care and custody does not change simply because they are in segregation."

A prisoner has never fallen ill or died because of being held in a segregation box, Fisher said.

Human rights advocates say little is known about how the military treats prisoners inside these boxes.

"There are concerns that they could be used in places where the detainees are enclosed in extremely hot conditions. It is important to know whether or not detainees are provided with food," said Jennifer Daskel of Human Rights Watch.

Prisoners get food and water during their time in the boxes, Fisher said.

Since the abuses at Abu Ghraib , the U.S. has improved conditions for the 20,000 prisoners there, the military said.

But life is tough behind the wire. Hundreds are still considered by the military to be al Qaeda loyalists.

The U.S. military has released about 10,000 Iraqi prisoners and said it hopes to release more. Recently, 20 foreign fighters were sent back to Egypt and Saudi Arabia, the military said.

"We are able to capture threats to the Iraqi government and the population, detain them, rehabilitate them, and 99 times out of 100 release them," Pentagon spokesman Geoff Morrell said.