When U.S. troops gave up their posts in Iraqi cities this summer, it turns out that's not all they gave up — at least in the eyes of Iraqi officials.

The Iraqi government, in a strict application of its security agreement with the United States, has placed additional restrictions on the movement of U.S. troops, according to a report by the Washington Post.

The Post cites what it calls a "curt missive" dated July 2 from the Baghdad Operations Command ordering U.S. military leaders to "stop all joint patrols" in the capital, carry out resupply missions only at night and notify Iraqi leaders immediately if there have been any violations of the security agreement.

U.S. forces withdrew from Iraqi cities at the end of June as a major step toward winding down the war there, though American soldiers remain stationed elsewhere in the country.

The strict application of the agreement coincides with Iraq's struggles to respond to an escalation in attacks by extremist groups backed by Iran, military officials told the Post.

Iraqi authorities showed some progress in taking on that threat when they announced Saturday they had arrested a member of an Iranian-backed militia suspected in an attack that killed three U.S. soldiers in southern Iraq.

Maj. Gen. Adil Daham, chief of the Basra provincial police, said the militiaman confessed early Saturday to the attack on a U.S. base near the airport. The soldiers were killed Thursday night in a rocket attack, the U.S. military said, in a rare assault on troops in the comparatively quiet south.

During a search of the house where the suspect and an aide were arrested, Iraqi officials said they seized four Iranian-made rockets and documents listing names of officials to be targeted.

U.S. military commanders believe some Shiite militias have received funds and training from Iran, which denies the charge.

Meanwhile, a bomb struck the vehicle of Sheik Naeim Salih al-Halbosi, a Sunni tribal leader, as he was leaving his house near Fallujah, a former insurgent stronghold west of the capital, police said.

The Associated Press contributed to this report.