Baghdad ? The top U.S. commander in Iraq warned his troops Friday to expect subtle changes in combat operations — including obtaining warrants before searching homes and detaining people — when the newly approved U.S.-Iraq security agreement takes effect Jan. 1.

American troops have already begun implementing some of the changes, such as conducting more joint operations with Iraqi soldiers and getting warrants before raids against suspected insurgents.

Iraq’s three-member presidential council signed off on the agreement Thursday, the final legal hurdle to enable the pact to go into effect next month — even though voters will have the final say in a referendum by the end of July.

It replaces a U.N. mandate that gives the U.S.-led coalition sweeping powers to conduct military operations and detain people without charge if they were believed to pose a security threat. The new pact requires U.S. troops withdraw from Baghdad and other cities by the end of June and leave the country entirely by January 1, 2012.