Guerrillas unleashed a coordinated assault on military bases and the governor's office in the southern city of Kerbala (search) on Saturday, killing 13 people — including six soldiers from the U.S.-led occupation force and six Iraqi police officers — and wounding at least 172, officials said.

Four of the dead soldiers were from Bulgaria and two from Thailand. An Iraqi civilian also was killed.

Attackers detonated four suicide car bombs and fired mortar shells and grenades, wounding at least 37 other coalition soldiers, including five Americans and 19 Bulgarians, U.S. Army Brig. Gen. Mark Kimmitt (search) said in Baghdad.

Some 135 Iraqi civilians and police officers also were wounded, said Ali al-Arzawi, deputy director of Kerbala General Hospital (search).

In other attacks Saturday, rebels detonated three homemade bombs that set aflame a fuel depot and injured six American soldiers.

Still, military officials said the number of attacks had decreased significantly. Kimmitt said attacks went down from about 50 a day in mid-September to an average of about 15 a day, spiking to 18 on Christmas Day.

He said this was a "technique to terrorize the people of Baghdad and of Iraq."

The U.S.-led administration, meanwhile, placed bounties of $1 million each on the heads of 12 fugitives from the list of the top 55 most-wanted Iraqis, officials said Saturday. There is a $10 million bounty on the head of the 13th remaining fugitive, Izzat Ibrahim al-Douri, a former senior official in the regime and a confidant of Saddam Hussein.

Abdul Aziz al-Hakim, current president of the U.S.-chosen Governing Council, said Saturday there was "nothing heroic about lobbing an RPG," or rocket-propelled grenade.

"It was a coordinated, massive attack planned on a big scale and intended to do much harm," Polish Maj. Gen. Andrzej Tyszkiewicz said from his headquarters at Camp Babylon, east of Kerbala.

Tyszkiewicz called the ambush the most serious attack suffered by coalition forces in the south-central part of Iraq. Poland commands a multinational force of 9,500 soldiers, including 2,400 Poles.

He said three car bombers were shot before they could drive the explosive-packed vehicles into the Bulgarian bases or the damage might have been more severe. The fourth bomb exploded in front of the regional governor's office.

Tyszkiewicz did not identify the nationalities of the dead and wounded.

But a Thailand government spokesman in Bangkok, Jakrapob Penkair, said he received reports that two Thai soldiers were killed by a car bomb.

A senior Thai army official told The Associated Press on condition of anonymity that the two were on guard duty at a military base checkpoint when the car rammed into the camp's wall.

Bulgaria's deputy defense minister, Ilko Dimitrov, said four Bulgarian soldiers were killed and 27 were wounded when a car bomb exploded at one of the Bulgarian camps. The 485 Bulgarian troops in the area were being evacuated because their bases had been destroyed, and the defense ministry had lost communication with them, he said. The headquarters was also destroyed.

Two men were detained for the car bomb that blew up in front the governor's office, said Lt. Col. Tom Evans, deputy commander of the U.S. Army's 18th Military Police Brigade.

A reporter saw three bombed-out cars on the street, which was sealed off by U.S. troops. A policeman at the scene said he saw two rockets explode in the street in front of the governor's office.

At Kerbala General Hospital, crying people crowded the corridors, searching for missing family members.

Mohamed Jassim, 50, was about to enter the governor's office when the bomb detonated.

"I was knocked out on the floor by the explosion," he said, nursing an injured hand. "I accuse (Saddam Hussein's) Baathists and Saddam's supporters of conducting this operation. Only they can do such criminal acts, targeting civilians."

Al-Arzawi, the deputy director of Kerbala hospital, said the Iraqi death toll included six police officers and a woman living next to one of the military bases.

The Polish general said rapid response forces and 10 helicopters were deployed to hunt the attackers.

Around 5 p.m. Saturday, a bomb was found and defused outside Kerbala's human rights organization, its director, Seyed Hussein Ibrahimi, said.

On Friday, four U.S. troops were killed in bomb blasts and a mortar attack and another died in a traffic accident, bringing the death toll for coalition forces in Iraq this week to 15.

Air Force Capt. Patricia Teran-Matthews, a U.S. military spokeswoman, said five soldiers were injured Saturday when a U.S. convoy hit a "daisy chain" of two homemade roadside bombs in Baghdad, and another soldier was hurt in a similar roadside blast at Habaniyah, west of the capital.

In the northern city of Mosul on Saturday, witnesses said rebels fired on a U.S. convoy from a car and then soldiers pursued them, shooting and killing four of them. The military did not confirm that report.

Also in Mosul, an Iraqi lawyer working on a project funded by the U.S.-led coalition was assassinated Saturday. Adil Hadidi was shot dead as he walked out of his house, according to a neighbor, Firas Kamal.

On Friday, gunmen in Mosul killed a Sunni Muslim tribal leader who backed the coalition. Anti-U.S. guerrillas have targeted Iraqi police and other officials cooperating with the U.S.-led occupation authorities.

Also Friday, three insurgents opened fire on a police station in Ramadi, some 60 miles west of Baghdad. One officer and one of the attackers were killed in an ensuing gunfight, police Col. Ziad Khalil said. The other rebels escaped.