~ The explosion took place in a neighborhood bordering the Green Zone in western Baghdad.

BAGHDAD -- A suicide car bomber struck in one of the capital's most heavily guarded neighborhoods Tuesday, killing two guards at a checkpoint near the home and offices of two prominent politicians, including the first prime minister after Saddam Hussein.

Both politicians were out of the country at the time.

The explosion took place in a neighborhood bordering the U.S.-protected Green Zone in western Baghdad, less than a quarter-mile from buildings that included the home and office compound of Ayad Allawi, a secular Shiite, and offices of Saleh al-Mutlaq, the head of the Iraqi National Dialogue Front, a Sunni political bloc.

It was the second bombing in two days to strike guards of Allawi, who is on a short list of possible future national leaders and a fierce critic of Shiite Prime Minister Nouri al-Maliki.

On Monday, police said a roadside bomb targeted a vehicle carrying guards for Allawi in the same neighborhood. Two guards were wounded, as were three policemen and a civilian, officials said.

Although the attacks caused few casualties, the fact that they occurred at all in a relatively secure part of the city is troublesome. U.S. troops have managed to shut down numerous car bomb factories around the city, reducing the number and intensity of bombings in recent months.

But U.S. commanders have warned that security in the capital is still fragile, despite marked improvements since last summer.

The suicide bomber detonated his explosives Tuesday at the first checkpoint along a street which includes both politicians' offices.

Al-Mutlaq, speaking from Amman, Jordan, said the bomber "claimed that he was an employee and had access." But police said the vehicle was speeding toward the checkpoint and the guards opened fire.

Al-Mutlaq confirmed reports by police and hospital officials that two guards were killed.

The offices are in a residential neighborhood, with many homes converted to work spaces. The area is convenient to the Green Zone, where the U.S. Embassy is housed and the Iraqi government has its headquarters.

"Everyone is vulnerable," he told Al-Arabiya television. "We have been targeted by three groups -- the Americans, Iraqi forces and a suicide bomber. Everyone should wake up and do something to change this situation."

In January, six Iraqis were killed in a U.S.-led raid on other offices for al-Mutlaq. The U.S. military and Iraqi police said they suspected the offices were being used as an al-Qaida safe house.

Hussam al-Azawi, a member of Allawi's party, said there were indications of an assassination plot ahead of the suicide attack.

"The threats and plots came from a neighboring country," al-Azawi said without elaborating. "We received intelligence about this and informed the government and the Americans to reinforce the guards at our headquarters."

In a statement, Allawi's Iraqi National Accord bloc also said it had informed the U.S., the U.N. and the Iraqi government of a plot against the former prime minister. "Unfortunately no action was taken," the statement said.

Also Tuesday, an anti-al-Qaida Sunni tribal sheik who was promoting national unity was killed along with his nephew in a drive-by shooting near Tal Afar, 260 miles northwest of Baghdad. The attack was the latest in a series of strikes against Sunnis who have joined forces with the American and Iraqi governments against the terror network.

In the southern city of Basra, the bullet-riddled bodies of a Christian woman and her brother were found in a garbage dump on Monday, police and church officials said Tuesday, speaking on condition of anonymity because they feared reprisal. The victims had been kidnapped the day before.

Basra's police chief, Maj. Gen. Jalil Khalaf, has said patrols of motorbikes or unlicensed cars with tinted windows are accosting women not wearing traditional dress and head scarves, known as the hijab, and the mutilated bodies of 40 women have been found this year.

The attacks in Iraq's second-largest city, 340 miles southeast of Baghdad and near the Iranian border, come with Britain poised to hand over security responsibilities for the Basra province to the Iraqis, probably next week.

Still, the U.S. military has pointed to strong security gains in Iraq, especially in the capital. It said Tuesday that mortar and rocket attacks in Baghdad had declined to 25 November from 49 in October. According to the statement, there were seven mortar or rocket attacks in Baghdad during the first week of December -- all but one in residential neighborhoods.