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 on 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.

In a letter to the nearly 150,000 troops, Gen. Ray Odierno sought to reassure members of his command that the new agreement would not diminish their ability to defend themselves, even though new rules spelling out when, where and how soldiers can open fire will be published.

"The new environment, though, will require a subtle shift in how we plan, coordinate and execute combat missions throughout Iraq," Odierno said. Under the agreement, the U.S. troops must get Iraqi approval for combat operations and carry them out "by, with and through the Iraqi security forces," he added.

Nevertheless, Odierno stressed that the coalition must "maintain our effectiveness in accomplishing our objectives," including combating al-Qaida and other insurgent groups. But he said "we must do so with respect for the Iraqi Constitution and laws, and we must continue to treat all Iraqi citizens with the utmost dignity and honor."

Odierno said the new rules of engagement — which among other things spell out when soldiers can open fire — will not diminish "our fundamental ability to protect ourselves and the force."

He said senior officers were in talks with the Iraqi government to work out procedures and that detailed orders, including new tactics, would be issued later.

"We will implement the agreement through phased, deliberate steps that preserve security gains, and we will complete our mission with honor and success," he said.

Nonetheless, the agreement will bring fundamental changes in the way U.S. forces operate here as the nearly six-year-long American mission winds down.

Among other things, the agreement states that after Jan. 1, U.S. troops may not search homes or businesses without warrants "except in the case of active combat operations." The U.S. military must transfer the more than 15,000 detainees in its custody to the Iraqis or release them if there is not enough evidence to hold them.

In preparation for the change, U.S. officers have been quietly implementing some new procedures. Already, most combat operations are conducted jointly with Iraqi soldiers and police, and more and more raids are carried out with warrants issued by Iraqi judges.

It is unclear, however, whether the changes will be as seamless as senior officers insist.

In the past, Prime Minister Nouri al-Maliki had been openly critical of some U.S. operations, including attacks against Shiite militias in Baghdad's Sadr City neighborhood until he broke this year with radical cleric Muqtada al-Sadr.

During a news conference Wednesday, the second-ranking commander in Iraq, Lt. Gen. Lloyd Austin, acknowledged there could be friction between U.S. and Iraqi officers on when and where to launch attacks.

"It's combat and you know there will be friction from day to day and we hope that these friction points will be minor," Austin said.

The level of fighting in Iraq has dropped significantly since a cease-fire last spring between the Iraqi government and Shiite militias in Basra and Baghdad — a move that enabled the Iraqi army to take control of flashpoint neighborhoods. Last month, only eight U.S. soldiers were killed in action, a fraction of the monthly toll earlier in the war.

At least 4,209 members of the U.S. military have died in the Iraq war since it began in March 2003, according to an Associated Press count.

Most of the fighting is taking place in the north against al-Qaida and other Sunni extremists who have suffered major setbacks but have not been defeated. On Thursday, two U.S. soldiers were killed by a suicide car bomber in Mosul, the main city in the north.

Attacks still occur in Baghdad and other areas, although at lower frequency.

Two truck bombers killed at least 17 people Thursday in Fallujah, a former Sunni insurgent stronghold 40 miles west of Baghdad. Police said two bodies — a policeman and a child — were found Friday in the rubble of nearby buildings destroyed by the blasts.

On Friday, a roadside bomb exploded near an Iraqi security patrol in southern Baghdad, killing one policeman and a Sunni volunteer, police said. Two other members of the patrol were wounded.

To the north, three women were killed in Balad Ruz when a bomb planted in a radio exploded, Iraqi officials said.

One of the women picked up the radio off the street and brought it home. It detonated when one of the women tried to turn it on, officials said.