The violence came a day after insurgents shot down a U.S. transport helicopter west of Baghdad, killing 16 Americans and wounding 21 in a dramatic show of increasing sophistication in their attacks.
Only two days before that strike, leaflets seen in mosques in the Fallujah area warned of new attacks using "modern and advanced methods."
In Baghdad, five strong explosions were heard in quick succession at about 9:10 p.m., and it appeared the blasts were coming from the western side of the Tigris River.
Soon after, the U.S military command reported three or four mortar impacts in central Baghdad. It did not report any casualties or give details on damage.
In other developments:
The Karbala blast occurred on a busy street less than 100 yards from the gold-domed Imam Hussein shrine in the city, 65 miles south of the Iraqi capital, 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. It was not immediately possible to get confirmation of the report from Iraqi police or the U.S.-led coalition.
Karbala was rocked by deadly clashes last month between supporters of rival Shiite factions. Also in October, supporters of a little-known Shiite cleric based in Karbala killed three U.S. soldiers in a rare clash between residents of the holy city and the U.S. military.
On Aug. 29, a car bomb exploded outside a mosque in the nearby city of Najaf, killing more than 85 people including Shiite leader Ayatollah Mohammed Baqir al-Hakim. It was the bloodiest incident since the end of the U.S.-led invasion that overthrew Saddam Hussein.
Sunday's downing of a U.S. Chinook helicopter outside Fallujah was the deadliest since strike against American forces since the U.S.-led invasion of Iraq in March.
Three other Americans were also killed Sunday in separate attacks, making it the bloodiest day for U.S. forces since March 23.
At the Chinook crash site, near the Euphrates River farming village of Hasi, just south of Fallujah, a giant crane lifted pieces of wreckage onto a truck. Soldiers sealed off the immediate area, deep in the "Sunni Triangle" that has produced the most violent opposition to the U.S. occupation of Iraq.
Witnesses said the attackers used shoulder-fired missiles against 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 large twin-rotor helicopter was flying with a second Chinook headed for Baghdad International Airport when two missiles streaked into the sky and slammed into the rear of the aircraft, witnesses told The Associated Press. It crashed in flames in farmers' fields west of Baghdad.
An unsigned leaflet posted Friday at mosques in the area urged people to avoid public places over the weekend. "Special operations against occupation forces might be carried out by using modern and advanced methods," the leaflet said.
The leaflet also warned people stay at home, avoid going to work or school and stay away from markets Saturday and Sunday. "Any persons who move during this period will be responsible for their own safety," the note said.
U.S. officials have blamed Saddam Hussein loyalists, foreign fighters and Islamic extremists for the stepped up attacks on the U.S. occupation.
On Monday, 16 U.S. soldiers wounded in the helicopter attack arrived in Germany for treatment at an American military hospital. Eleven of the soldiers were in intensive care but stable condition, while five others were in the main ward with lesser injuries, said U.S. Army Col. Rhonda Cornum, commander of the Landstuhl Regional Medical Center.
The soldiers — 15 men and one woman — were among nearly 30 soldiers who arrived early Monday at Ramstein Air Base aboard a C-17 transport aircraft. A 17th soldier injured in the attack was to arrive Tuesday morning, said Marie Shaw, a spokeswoman for the hospital.
In Britain, Prime Minister Tony Blair expressed his regret over the shootdown, saying the past week had been a "particularly grim seven days" in terms of casualties, a spokesman said Monday.
L. Paul Bremer, the head of the occupation in Iraq, repeated demands that Syria and Iran prevent fighters from crossing their borders into Iraq.
"They could do a much better job of helping us seal that border and keeping terrorist out of Iraq," he said in a broadcast interview. The "enemies of freedom" in Iraq "are using more sophisticated techniques to attack our forces."
U.S. officials have been warning of the danger of shoulder-fired missiles, hundreds of which are now scattered from Saddam's arsenals, and such missiles are believed to have downed two U.S. copters since May 1. Those two crashes — of smaller helicopters — wounded only one American.
The insurgents have been steadily advancing in their weaponry, first using homemade roadside bombs, then rocket-fired grenades in ambushes on American patrols, and vehicles stuffed with explosives and detonated by suicide attackers.
With the helicopter crash, at least 139 American soldiers have been killed by hostile fire since 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.
The violence continued late Sunday, when five shells exploded in different neighborhoods in the northern oil city of Kirkuk, killing one Iraqi and injuring eight, Jalal Jawher, a Patriotic Union of Kurdistan official said on Monday.
On Saturday, fire broke out at an oil pipeline in Samara, 60 miles northwest of Baghdad after it was hit by a bomb, an oil official said under condition of anonymity. The pipeline runs between northern Kirkuk to a Baghdad refinery.
A second explosion occurred Saturday on the pipeline between Saddam's hometown of Tikrit and Beiji, the site of Iraq's largest refinery on an LPG line that links Kirkuk to Taji gas factory in Baghdad.
Constant sabotage to pipelines and the decayed state of Iraqi's infrastructure have slowed efforts to revive the country's giant oil industry.