Strong explosions were heard late Monday in central Baghdad, and it appeared the blasts were coming from the western side of the Tigris River.
The U.S. military command said three to four mortars impacted in the center of the capital but gave no details about where the explosions occurred precisely or whether there were casualties or damage.
Meanwhile, a blast near a holy Shiite Muslim shrine in the city of Karbala killed three people and injured 12, witnesses said.
It was not immediately possible to get confirmation of the report from Iraqi police or the U.S.-led coalition.
The blast occurred on a busy street less than 100 yards from the gold-domed Imam Hussein shrine in the city 65 miles south of Baghdad, said Mohammed Abu Jaffar al-Assadi, a Shiite cleric.
All the dead apparently were passers-by. He said the bomb had apparently been planted in a parked car.
Karbala was last month rocked by deadly clashes between supporters of rival Shiite factions. Three soldiers died there last month, and 85 people perished in a car bombing in August.
The attacks come a day after the downing of an Army helicopter that killed 16 U.S. soldiers, on a day in which three other Americans, including two civilian contractors, also were killed in Iraq. The attack was the single deadliest event of the war for U.S. troops, which began in March.
In other developments:
The U.S. deaths Sunday brought to at least 139 the number of American soldiers killed by hostile fire since Mr. Bush declared an end to combat on May 1.
Around 377 U.S. service members have died since the beginning of military operations in Iraq. Daily attacks against U.S. forces have increased in the last three weeks from an average of the mid-20s to 33.
Sunday's missile attack, which also wounded 20 troops, closed out a week that began with a similarly grim new record. On Oct. 27, three dozen people died in a wave of suicide bombings in Baghdad, the bloodiest day there since President Bush declared major combat over May 1.
Witnesses said the attackers used missiles to down the Chinook — a sign of the increasing sophistication of Iraq's elusive anti-U.S. fighters. As a result, CBS News Reporter Charles D'Agata reports, U.S. commanders have suspended daylight flights by Chinooks.
Mr. Bush, spending a long weekend at his Texas ranch, said nothing in person about the helicopter shoot-down Sunday,
But White House spokesman Trent Duffy, in a statement read to reporters, said: "The terrorists seek to kill coalition forces and innocent Iraqis because they want us to run, but our will and resolve are unshakable."
The White House statement sought to remind Americans that Mr. Bush sees military action in Iraq as tied to the Sept. 11 terror attacks — and as part of a larger battle to head off future attacks.
"Sept. 11 taught us that we must confront terrorists and outlaw regimes with weapons of mass murder before it is too late," Duffy said. "The only way to win the war on terror is to take the fight to the enemy."
Sen. Joseph Biden, the ranking Democrat on the Senate Foreign Relations Committee, told the CBS News Early Show that the United States must finish the job it started in Iraq.
However, Biden criticized the Bush administration's war effort for lacking a "sense of urgency" in securing the peace and said more troops are needed for the job.
The United States, he said, needs to "bring in NATO, bring in other folks and give up some authority. We act like Iraq is some kind of prize that we won."
Speaking Sunday, Defense Secretary Donald H. Rumsfeld said Americans should view the deadly downing of the Army helicopter as the tragic but inevitable cost of waging a long war.
"In a long, hard war, we're going to have tragic days, as this is," he said. "But they're necessary. They're part of a war that's difficult and complicated."
The downed copter was one of two Chinooks flying out in formation from an air base in Habbaniyah, about 10 miles from the crash site, carrying troops to Baghdad enroute to long-awaited two week-long leaves from combat duty.
The U.S. military would not confirm that the aircraft was struck by a missile, but a spokesman, Col. William Darley, said witnesses reported seeing "missile trails."
Like past attacks on U.S. forces and a string of suicide bombings that killed dozens in Baghdad the past week, U.S. coalition officials blamed either Saddam loyalists or foreign fighters for the strike outside Fallujah, a center of Sunni Muslim resistance to the U.S. occupation.