An Iraqi soldier opened fire on a U.S. military team Saturday, killing two American soldiers and wounding three, the U.S. military said, in an attack that could sharpen worries about the extent of militant infiltration in Iraq’s security forces.

Iraqi officials described the attacker — who was killed in the gunbattle — as a soldier who also served as a Sunni Muslim preacher for his unit near Mosul, which is one of the last urban strongholds for Sunni insurgents.

Such an ambush could increase pressure on the Shiite-led government to try to root out possible turncoats and slow efforts to bring Sunni militiamen into the police and military as rewards for helping battle al-Qaida in Iraq and other insurgent factions.

But any possible slowdown of the Sunni outreach will meet resistance from Washington, which sees the sectarian reconciliation as essential for Iraq’s stability and to keep security gains from rolling back.