A roadside bomb exploded Thursday in eastern Baghdad, killing four U.S. soldiers and wounding two others, the U.S. military said, and an undetonated device believed to be from Iran was found at the site.

The attack began when one bomb went off as a U.S. unit was returning from a search operation in the mostly Shiite area, the military said. Moments later, a second bomb exploded, killing and wounding the soldiers.

A demolition team that searched the site after the attack found an explosively formed projectile, a type of high-tech bomb which the U.S. military believes comes from Iran. The device was detonated by the team.

Earlier Thursday, the military said a U.S. soldier was killed the day before in combat in Anbar province, west of Baghdad.

In addition, a Marine assigned to Multi-National Force-West also died Wednesday in a non-combat related incident in Anbar, the military said in a separate statement. It did not give more details, saying the incident is under investigation.

Identities were not released pending notification of relatives.

In Thursday's violence, a homicide car bomber apparently targeting a senior city official struck an Iraqi military checkpoint Thursday in a Shiite neighborhood in Baghdad, killing at least eight people, officials said.

The blast came a day after the U.S. military said stopping car bombings had become the main focus of a major security sweep to halt the sectarian violence in the capital. Karradah, a major commercial district, has been hit by bombings several times in recent months, including a suicide car bombing on Jan. 25 that killed 30 people.

In the city of Iskandariyah, 30 miles south of Baghdad, a bomb in a parked car exploded as a bus packed with workers passed by, killing at least four and wounding 24, police said.

A U.S. soldier was killed Wednesday in fighting in Anbar province, a Sunni insurgent stronghold west of Baghdad, the military said. A Marine assigned to Multi National Force-West also died the same day in Anbar in a non-combat-related incident that is under investigation, the military said separately.

The deaths raised to at least 3,202 members of the U.S. military who have died since the war began four years ago this month, according to an Associated Press count.

The top official in Baghdad's main Shiite district of Sadr City was seriously wounded when gunmen ambushed his convoy in eastern Baghdad, killing two of his bodyguards, according to police and a local official.

Rahim al-Darraji has been involved in negotiations with U.S. and Iraqi government officials seeking to persuade the Shiite militias that dominate the sprawling slum to pull their fighters off the streets ahead of a security crackdown to stop the sectarian warfare in Baghdad.

His convoy was attacked in a drive-by shooting in the mostly Shiite area of Habibiyah.

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In another development, a judge said the death sentence for Saddam Hussein's former deputy Taha Yassin Ramadan had been upheld on appeal, and the former vice president will be hanged for his role in the killing of 148 Shiites in 1982.

The Baghdad blast, which occurred just after 3 p.m., tore through a popular square in the predominantly Shiite area of Karradah, sending a huge plume of black smoke rising above the flags atop the nearby French Embassy. Officials said eight people were killed and 25 were wounded.

The driver detonated the explosives as a convoy carrying the head of the city council, Sabir al-Issawi, was passing the checkpoint on Kahramanah Square, the council chief's deputy told the AP. The mayor was unhurt, but three of his bodyguards were wounded, Naeem al-Qabi said.

The bombing in Iskandariyah occurred about 7:30 a.m. as the bus was carrying employees of a state-owned car company to work. It shattered the facades of nearby houses, leaving piles of concrete rubble along the street. A ball of crumpled metal over a tire was all that was left of the car that exploded.

Ali Jassim Mohammed, a 40-year-old government employee who was about 500 yards away, said he saw "a ball of fire next to the bus."

The bombing came a day after gunmen attacked a Sunni mosque in the city, causing no casualties but severely damaging the building. Iskandariyah is located in an area dubbed the "Triangle of Death" because of relentless attacks by Sunni insurgents.

On Wednesday, U.S. and Iraqi officials issued cautiously optimistic reports on the security clampdown in Baghdad, but they warned that months would pass before the operation could be labeled a success.

Maj. Gen. William B. Caldwell, the U.S. military spokesman, said indicators from the Iraqi government have been "extremely positive."

"But I would caution everybody about patience, about diligence. This is going to take many months, not weeks, but the indicators are all very positive right now," he added.

Caldwell said the level of sectarian killings had dropped significantly since the operation began but he expressed concern about a spike last week in the number of what he called "high-profile" car bombings.

"If the high-profile car bombs can be stopped or brought down to a much lower level, we'll just see an incredible difference in the city overall," he said.

Brig. Gen. Qassim al-Moussawi, spokesman for the Baghdad security operation, said Wednesday the number of civilians killed had plunged to 265 since the operation started Feb. 14, compared with 1,440 during the preceding monthlong period, due to a sharp reduction in murders, kidnappings and bombings.

Judge Mounir Haddad said the decision on Ramadan's execution was relayed to Prime Minister Nouri al-Maliki's government, which will set the date. Haddad, a member of the court's nine-judge panel, said the decision to uphold the death sentence was unanimous.

The panel ignored appeals from international human rights groups. Human Rights Watch and the International Center for Transitional Justice said the evidence was insufficient for such a punishment.

Ramadan was convicted in November along with Saddam and six others in the killings of Shiites in Dujail following an assassination attempt against the former Iraqi leader in 1982 in the Shiite town of Dujail north of Baghdad. Three other defendants were sentenced to 15 years in jail in the case, while one was acquitted.

Ramadan was sentenced to life in prison but an appeals court ruled that was too lenient and asked that the lower court reconsider. The court sentenced him to death last month.

He maintained his innocence, saying his duties were limited to economic affairs, not security issues.

Saddam was hanged Dec. 30 and two of his co-defendants in the Dujail case — his half brother and former intelligence chief Barzan Ibrahim, and Awad Hamed al-Bandar, former head of Iraq's Revolutionary Court — were executed in January.

The government, meanwhile, announced that it had decided to hold a minute of silence in all Iraqi cities on Friday morning to commemorate the anniversary of the 1988 chemical weapons attack on the Kurdish town of Halabja in northern Iraq, according to state TV. Saddam had ordered the attack as part of a scorched-earth campaign to crush a Kurdish rebellion in the north, seen as aiding the Iranian enemy.

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