BAGHDAD, Iraq – A deadly gun battle broke out between rival Sunni insurgent factions near the northern city of Samarra, in fresh violence that highlighted the profound division among these organizations and complicated nature of the fighting in Iraq.

The violence began Friday night outside Samarra, north of Baghdad, between members of the Islamic Army and al-Qaida in Iraq, two Sunni insurgent groups that have become enemies. Many members of the Islamic Army have rejected al-Qaida in Iraq’s methods and aligned themselves more closely with American soldiers, according to Iraqi police and insurgents.

Islamic Army fighters killed 18 suspected members of al-Qaida in Iraq and captured 14 others, according to a leader of the Islamic Army known as Abu Ibrahim.

“Yes, there were heavy clashes that started last night, but we did not intervene because it was between the resistance factions,” said Maj. Bakir al-Bazi of the Samarra police. “But we will try to open channels with the Islamic Army to fight al-Qaida.”

Abu Ibrahim said the fighting was in response to attacks by al-Qaida in Iraq that killed four Islamic Army leaders, including one who was kidnapped from his house and killed in a public bazaar.

He denied that anyone from his organization was killed in the most recent fighting, though an Iraqi police source told the Reuters news service that 15 Islamic Army fighters were killed.

The clashes erupted again Saturday afternoon, Abu Ibrahim said, and two more al-Qaida in Iraq leaders were killed.

A U.S. military spokesman in northern Iraq said he had no information about any fighting in the area.

Over the course of the year, several Sunni insurgent groups have taken up the fight against al-Qaida in Iraq. Many of them have joined the ad hoc forces organized by U.S. soldiers to serve as local defense groups, providing intelligence information and other assistance to the Americans about their common enemies. The Iraqi government has expressed concern that some of these fighters have abused their new authority and has said it fears emboldening new, unaccountable militias.

On Saturday, members of these “volunteer” forces in Diyala province, working with U.S. and Iraqi troops, helped arrest as many as five leaders of al-Qaida in Iraq, said Hussain al-Zubaidi, an official with the Diyala Provincial Council.

The U.S. military issued a statement saying 10 people were arrested Saturday during operations in central and northern Iraq to disrupt al-Qaida in Iraq.

In a separate development, the U.S. military said a U.S. soldier was killed, and three others were wounded, when a bomb exploded near them Friday in Diyala province.