Four more U.S. soldiers were killed in roadside bombings in the Baghdad area, including three in a single strike, the military said Tuesday.

The violence came as Iraqi authorities intensified checkpoints and announced plans for a vehicle ban beginning Wednesday night in Baghdad as they girded for a major Shiite pilgrimage to commemorate the eighth-century death of an important Shiite saint.

Iraq's political crisis also worsened as five ministers loyal to former Iraqi leader Ayad Allawi announced a boycott of Cabinet meetings — leaving Prime Minister Nouri al-Maliki 's unity government with no members affiliated with Sunni political factions.

The Iraqi List expressed frustrations that its demands had been ignored but did not impose a deadline.

"We have to warn all political groups ... that the dangers are real and our people are suffering and bleeding," the group said. "Leaving things as they are now will mean more destruction for our people."

Al-Maliki, who arrived in Turkey for a state visit, dismissed the criticism of his leadership.

"This will not affect the government," he told The Associated Press in an interview aboard the plane en route to Ankara.

In Ankara, al-Maliki agreed to a Turkish request try to root out a Kurdish rebel group from northern Iraq.

"We have reached an agreement to spend all efforts to end the presence of the Kurdistan Workers Party or PKK in Iraq," Turkish Prime Minister Recep Tayyip Erdogan said at a joint news conference with al-Maliki.

Erdogan said the leaders signed a memorandum of understanding and agreed to speed up work to finalize a counterterrorism agreement to combat the Kurdish guerrillas who have escalated their attacks on Turkey from bases in northern Iraq.

Saudi Foreign Minister Prince Saud said that a delegation will head to Iraq next week to explore the reopening of the kingdom's embassy in Baghdad.

The trip will be the kingdom's first concrete step toward opening a diplomatic mission in Iraq, a measure the U.S. administration has sought from Iraq's neighbors as a sign of support for the government in Baghdad.

From around Iraq, thousands of women shrouded in black and men wearing traditional white Arab robes have begun the trek toward the shrine of Imam Moussa ibn Jaafar al-Kadhim, one of 12 principal Shiite saints. He was said to have been buried where golden-domed Moussa al-Khadim mosque now stands in the northern neighborhood of Kazimiyah. The main procession was expected to reach the house of prayer on Thursday.

Sunni insurgents often target such gatherings. And this pilgrimage was struck by tragedy in 2005, when thousands of Shiite pilgrims, panicked by rumors of a suicide bomber, stampeded on a bridge. An estimated 1,000 people died.

Sami Faraj, a 52-year-old government employee, said he would set out with his wife and children for the two-hour walk from the central neighborhood of Karradah to Kazimiyah on Wednesday, then they would help distribute food, pastries, tea and ice cream to people in honor of the imam.

"We do not care about the bombings and the terrorists. We are ready to sacrifice ourselves for the cause and for the sake of the prophet's descendants," he said.

Brig. Gen. Qassim al-Moussawi, a military spokesman for Baghdad, said all cars, trucks, motorcycles and carts would be banned from moving in the streets of Baghdad from 10 p.m. Wednesday until 5 a.m. Saturday. The ban would begin at 10 p.m. Tuesday in Kazimiyah, where Iraqi security forces already were deployed in force, he added.

"We have information that Takfiri and terrorist groups will try to disturb this occasion by targeting the innocent citizens and their processions," al-Moussawi said, using terms the government applies to Sunni extremists.

The attacks against U.S. forces included two powerful roadside bombs that left multiple soldiers dead and wounded — three soldiers died Saturday south of Baghdad and four were killed Monday in a blast that also wounded 11 in restive Diyala province north of the capital.

One soldier also was killed and another wounded Monday when their vehicle was targeted by an armor-piercing explosively formed penetrator, or EFP, in a western section of the capital, according to a separate statement.

Washington has accused Iran of supplying Shiite extremists with EFPs. Tehran denies the allegations.

The U.S. troop deaths increased to at least 19 so far this month, or a rate of about three per day, putting August on track for a heavier toll after a drop in July. Seventy-nine American troop deaths were reported in July, the lowest number since 70 killed in November.

More than 100 American forces died each month in the April-June period as the incoming U.S. troops were deployed with the Iraqi army in Baghdad's dangerous streets and security outposts.

Despite the relatively low number in July, Lt. Gen. Raymond Odierno, the U.S. second-in-command, said rogue Shiite militia fighters allegedly armed and trained by Iran were responsible for nearly three-quarters of attacks that killed or wounded his soldiers. He said Tehran was increasing its support for militants ahead of a pivotal report on progress in Iraq to be delivered to Congress next month.

The Iraqi List boycott left the government, at least temporarily, without participants who were members of the Sunni political apparatus — a deep blow to the prime minister's attempt to craft reconciliation among the country's majority Shiites and minority Sunnis and Kurds.

The defense minister is from a Sunni background but has no political ties and was chosen by al-Maliki.

The Allawi bloc, a mixture of Sunnis and Shiites, cited al-Maliki's failure to respond to its demands for political reform. The top Sunni political bloc already had pulled its six ministers from the 40-member Cabinet of al-Maliki, a Shiite, last week.

"This decision is a bid to apply political pressure for reforming the political process that is headed in the wrong direction," bloc spokesman Ayad Jamaluddin said Tuesday.

He declined to give a deadline but said the bloc's demands included reconsidering efforts to revise legislation to bring thousands of former Saddam Hussein-era party officials back into the government and purging the security forces of extremists.

The ministers intend to continue overseeing their ministries.

A Syrian rights group warned that the number of Iraqi refugees who have come to this country is now almost at 2 million, with many of them facing a desperate situation. According to a report by theSyrian Organization for Human Rights, the number of Iraqis who fled the spiraling violence at home to seek shelter in Syria has topped 10 percent of Syria's population of 18.5 million.

Complete coverage is available in FOXNews.com's Iraq Center.