A United Nations official says Iraqi militias are recruiting children to kidnap, kill, and even serve as suicide bombers.

The U.N. special representative for children and armed conflict, Radhika Coomaraswamy, said Friday that only 50 percent of Iraq's primary school-age children are in school. She said increasing numbers of children are being recruited to work for insurgent groups, where they become what she called the "silent victims of the continued violence."

Meanwhile, the U.S. military says an explosion that started a huge fire at an oil pipeline south of Baghdad Friday was an accident. Iraqi police initially said the blast was caused by a bomb. Eight people were wounded in the blaze.

Earlier Friday, radical Shi'ite cleric Moqtada al-Sadr urged his Mahdi Army militia to stop fighting fellow Iraqis. Sadr said his recent call for "open war" is only against U.S. forces, not the Iraqi government.

Militias loyal to Sadr have been battling coalition and Iraqi forces in Baghdad's Sadr City district since late March. More than 300 people have been killed in those clashes.

Some information for this report was provided by AP and Reuters.