Bomb attacks in central Iraqi towns killed five American soldiers and four Iraqis on Saturday, a day after two U.N. security experts arrived in the capital to study the possible return of the world body's international staff.

The deadliest attack took place in Khaldiyah (search), west of the capital, where a four-wheel-drive vehicle rigged with explosives drove up to a U.S. checkpoint at a bridge and detonated, a witness said.

The U.S. military said three American soldiers were killed in the attack. Six soldiers and several Iraqi civilians were wounded, the military said.

About 20 miles away, near the town of Fallujah (search), a roadside bomb went off as a U.S. convoy passed, killing two soldiers.

The latest deaths brought to 512 the number of American service members who have died since the United States and its allies launched the Iraq war March 20.

The two bombings took place in towns in the heart of the so-called Sunni Triangle (search), the region north and west of Baghdad where the anti-American insurgency has been strongest. Despite Saddam Hussein's capture on Dec. 13, insurgents loyal to him have continued to attack police stations and U.S. troops.

In a third attack Saturday in the area, a truck bomb exploded soon after a U.S. patrol passed by in Samarra (search), killing four Iraqis and wounding 33 people, military spokesman Brig. Gen. Mark Kimmitt told reporters. Three American soldiers were slightly wounded, he said.

The American military police patrol was turning into a police station to join Iraqi police when the explosion occurred behind it, Sgt. Maj. Nathan Wilson of the 720th Military Police Battalion.

Also Saturday, at least one sniper in a building shot and wounded an American soldier who was in a foot patrol in a Baghdad neighborhood, Maj. Kevin West said.

A bridge across the Tigris River (search) in Baghdad, leading to the coalition headquarters, was closed by U.S. troops for two hours Saturday. Witnesses said they were searching for a bomb, but this could not be independently confirmed.

Baghdad has been a frequent target of insurgents. In one of the deadliest attacks, the U.N. headquarters in the capital was bombed in August, killing 22 people including top U.N. envoy Sergio Vieira de Mello. U.N. Secretary-General Kofi Annan withdrew all foreign U.N. staff in October.

A U.N. military adviser and a security coordinator arrived Friday in Baghdad, the first foreign staff to return since then.

They planned to meet with officials from the U.S.-led coalition and inspect buildings the world body might use, U.N. spokesman Stephane Dujarric said.

"Their primary focus will be to open lines of communication ... and also to look after the interests of our national staff in Iraq," Dujarric said.

Annan also is considering sending a separate security team that would be needed if he decides to send experts to Iraq to determine whether direct elections for a transitional government were feasible.

That team would help resolve a dispute between the coalition and Iraq's leading Shiite Muslim cleric, Grand Ayatollah Ali al-Husseini al-Sistani (search), who is demanding direct elections as opposed to a U.S. plan that calls for letting regional caucuses choose a legislature. The legislature would then name a new Iraqi government that will take over from the coalition July 1, under the U.S. plan adopted Nov. 15.

Abdul-Aziz al-Hakim (search), a Shiite leader, said Friday the plan "as it stands ... is unacceptable." But Americans and others are slowly coming around to the need for elections, he said.

Al-Hakim, who was among members of a Governing Council delegation that met with President Bush on Tuesday at the White House, heads the Supreme Council for the Islamic Revolution in Iraq, the country's most powerful Shiite political group.

He said if the U.N. experts conclude an early vote is not feasible, then sovereignty could be handed over to the U.S.-installed Iraqi Governing Council (search). But he added it was "a last-resort option."

Al-Hakim's views carry considerable weight in Iraq, where the Shiite majority has risen to dominate the political scene after decades of suppression by the Sunni Arab minority.

The United States maintains it is impossible to hold elections in such a short time given the lack of a census and electoral rolls and the continuing violence.

The Bush administration said Friday it was holding to its July 1 deadline for ending the U.S. occupation but the method of selecting a new government wasn't decided.

Elsewhere Saturday, some 7,000 Kurdish university teachers and students demonstrated in support of federalism outside Sulaimaniyah University in the largest Kurdish city of northeastern Iraq. The demand worries many who fear it will lead to Iraq's breakup into smaller states.

Most Iraqi Kurds, who comprise an estimated 15-20 percent of the country's 25 million people, live in northern provinces, which had enjoyed virtual autonomy under the protection of U.S. and British forces since 1991 following the first Gulf War.

However, Turkish, Syria and Iran fear that granting Kurds their own ethnic enclave could incite Kurdish minorities within their own borders.