Updated

Roadside bombs killed two U.S. soldiers and two Iraqi children in central Iraq, a day after strikes in the holy city of Karbala (search) targeted America's coalition partners and underscored how the insurgency remains strong despite Saddam Hussein's capture.

The Karbala attacks — multiple, coordinated strikes involving mortars, machine guns and a car bomb — claimed six more lives Sunday, raising the death toll to 19.

In central Baghdad (search) on Sunday, a bomb planted beside a road killed two Iraqi children and an American soldier and wounded 14 people, said U.S. Army Sgt. Patrick Compton of the Army's 1st Armored Division. The wounded include five American soldiers, their Iraqi interpreter and eight members of the Iraqi civil defense corps.

"It was a bad one," Compton said. "It's a real densely populated area of town."

Attackers detonated another roadside bomb later Sunday as a U.S. convoy was traveling on a road near Fallujah (search), west of Baghdad, killing another American soldier and wounding three, the U.S. military said. The latest deaths bring the number of American combat deaths since the invasion in March to 325.

Despite the attacks, U.S. military officials say the number of rebel assaults has dropped from about 50 a day in mid-September to an average of 14 or 15 a day, rising to 18 on Christmas Day. On Monday, American troops defused a bomb in the northern city of Mosul (search), an American soldier at the scene told a reporter as helicopters took to the air to search for the assailants.

Such bombs have been one of the insurgents' most effective tools striking against the coalition's overwhelming firepower.

The Karbala attacks on Saturday were the biggest since the Dec. 13 capture of Saddam Hussein (search). Bulgaria declared Tuesday an official day of mourning for six Bulgarian and two Thai soldiers killed in the attacks, which destroyed the Bulgarians headquarters in Iraq and appeared designed to undermine the resolve of U.S. allies soldiering in Iraq.

Six coalition soldiers, six Iraqi police officers and a civilian died Saturday. On Sunday, a Bulgarian lieutenant and five Iraqis wounded in the attacks died in the hospital, the Polish news agency PAP reported.

In the northern city of Kirkuk, police said they arrested four foreigners Sunday night who could be connected with attacks in that city. Police chief Saad al-Ubaidi said two Egyptians, one Afghan and one Iranian, who all had fake passports, were handed over to U.S. troops.

"They are strangers in the city ... believed to have links to the bombings in Kirkuk and its suburbs and to have links to the resistance" to the U.S.-led occupation, he told The Associated Press.

The military has said some foreigners have infiltrated from neighboring countries, though most guerrillas in Iraq are believed to be Saddam loyalists or militant Muslims.

The Kirkuk police chief also reported that overnight a lone gunman attacked the office of an organization working with the U.S.-led administration, and killed a police officer guarding the building.

Thailand announced Monday it will send an additional 30 troops to provide security for its other troops. Prime Minister Thaksin Shinawatra said Thailand would fulfill its one-year commitment to the United States and would "not run away from a friend."

Officials said Sunday they arrested five Iraqi suspects in the Karbala attacks. More than 130 Iraqis and several dozen coalition soldiers were injured, officials said, including five Americans and 26 Bulgarians. Six of Bulgarians are in serious condition, PAP reported.

The Thai and Bulgarian troops form part of a multinational force of 9,500 soldiers under Polish command.

Meanwhile on Monday, Japan pledged to forgive "the vast majority" of Iraq's nearly $8 billion debt if others do the same, giving a critical boost to the U.S. campaign for debt relief.