Some 700 U.S. troops have left nearby Baghdad to join 3,500 U.S. and 20,000 Iraqi soldiers already in Diyala, where the U.S. military says attacks on U.S. troops have risen 70 percent since summer.
U.S. officials say the rise is due to Sunni insurgents who fled Baghdad ahead of the U.S.-led security campaign there.
Despite the security operation, at least 23 people were killed or found dead in Baghdad on March 13, including four men shot dead in a Sunni mosque in the southwest of the city.
Elsewhere, Iraq's Prime Minister Nuri al-Maliki, a Shi'ite, traveled to the city of Al-Ramadi on March 13 on his first visit to the heartland of Sunni Arab insurgents fighting his U.S.-backed government.
Al-Maliki pledged to restore services in the vast province of Al-Anbar Governorate, scene of a four-year-old rebellion.