BAGHDAD - Iraq's three-member presidential council yesterday approved a timetable for withdrawal of all U.S. troops, removing the last legal barrier so that the agreement could take effect Jan. 1.

But the signing came on a day of increased violence. Bombings killed 20 people - including two American soldiers - and wounded more than 100 in a string of blasts in two Iraqi cities.

The brazen attacks in areas where the U.S. military has struggled for years to maintain order raised questions about Iraq's ability to ensure its own security as the United States scales down its own combat role under the newly ratified U.S.-Iraqi security pact, which calls for an American withdrawal from cities by June and from the country by Dec. 31, 2011.

The latest bombings underscore the fragility of Iraq's recent security gains, adding new urgency to U.S. efforts to train and equip an Iraqi security force capable of maintaining order after U.S. troops have gone home.

The two Americans were killed when a suicide driver detonated an explosive-laden car near an Iraqi checkpoint in the northern city of Mosul, military spokesman Lt. Col. Dave Doherty said. Iraqi police said eight people were wounded, most of them civilians.

The deadliest attacks occurred in Fallujah, the country's most heavily guarded city and once the symbol of Sunni Arab resistance to the U.S. occupation. Truck bombers struck within minutes of each other outside two police stations in different parts of the city, killing 15 people, wounding more than 100, and shattering nearby buildings, police and hospital officials said.

An al-Qaida front group, the Islamic State of Iraq, purportedly claimed responsibility for the attack.

Northeast of Baghdad, a bomb left on a parked motorcycle exploded near a restaurant in Baqubah, another one-time Sunni militant stronghold, killing three people and wounding 10, according to police.

U.S. commanders say that attacks are down 80 percent nationwide since March, but that al-Qaeda and other militants remained capable of staging limited but high-profile attacks.

Approval by the presidential council came one week after parliament signed off on the agreement, which was hammered out during months of tough negotiations that at times seemed on the point of collapse. President Jalal Talabani, a Kurd, and his two deputies Tariq al-Hashemi, a Sunni, and Adel Abdul-Mahdi, a Shiite, signed the accord at their headquarters in Baghdad.

President Bush called Prime Minister Nouri al-Maliki to thank him for his work on securing the pact, the White House said.

The agreement is still subject to approval by Iraqi voters in a referendum by the end of July. If voters reject the deal, Iraqi will ask the United States for a new round of talks.

Under the agreement, Iraq will gain strict oversight of the nearly 150,000 American troops now on the ground, representing a step toward full sovereignty for Iraq and a shift from the sense of frustration and humiliation that many Iraqis feel at the presence of American troops on their soil for so many years.