BAGHDAD: Five bomb attacks struck Shiite pilgrims Monday during an important religious ritual for the Muslim sect, killing 21 people and wounding nearly 100 others, revealing the enormous security challenges that still beset Iraq as the U.S. military leaves the country.

Shiite religious holidays such as the mourning period known as Ashoura are targeted every year by Sunni extremists and have become especially difficult tests for the U.S.-trained Iraqi security forces, still struggling to protect their citizens.

The U.S. military is rapidly shipping troops and equipment out of the country before a Dec. 31 deadline to have all its forces out - then the entire responsibility for the nation's security will rest with Iraq's leaders.

Further underscoring the shortfalls, assailants just a week ago were able to even get a car bomb into the heavily fortified Green Zone that is the government's headquarters in the capital and is home to many foreign embassies, prompting the U.S. mission to severely limit the movement of its staff inside the zone. Iraqi officials say the bomb was meant for Prime Minister Nouri al-Maliki, but he was not in the area at the time.

On Ashoura, the hundreds of thousands of pilgrims who walk to the southern holy city of Karbala from around the country in a show of religious Shiite fervor present a particularly easy target for Sunni militants who do not consider Shiites to be true Muslims.

After the fall of Saddam Hussein's Sunni-dominated regime in 2003, Shiites regained the right to express their beliefs freely, and since then the annual commemorations have drawn huge crowds despite the threat.

In Monday's first attack, a bomb exploded among Shiite pilgrims in Latifiyah, about 20 miles (30 kilometers) south of the capital, killing two of them and wounding three others, police said. A medical official confirmed the casualty toll.

Hours later, a car bomb exploded near a group of Shiite pilgrims in the town of Mahaweel as they were heading to Karbala, killing eight people and wounding about 56 other pilgrims, said police officials in Babil province.

Mahaweel is about 35 miles (56 kilometers) south of Baghdad.

Three more attacks against Shiite pilgrims in the capital killed 11 people and wounded 41 others, police said.

All officials spoke on condition of anonymity because the are not authorized to talk to the media.

Ashoura marks the anniversary of the seventh-century death of Imam Hussein, a grandson of the Prophet Muhammad. His death in a battle outside of Karbala sealed Islam's historical Sunni-Shiite split, which still bedevils the Middle East and Iraq in particular. To commemorate his death, Shiites walk from around the country to Karbala.

Security is usually very tight in and around the city, so insurgents have taken to attacking the pilgrims during their long walk to and from Karbala when its much harder for the Iraqi security forces to protect them.

Security has also been intensified in and around Baghdad's Green Zone, which is home to the U.S. Embassy, the Iraqi parliament and numerous government offices and official residences.

Last week's bombing there took place outside the parliament compound's main gates. Al-Maliki told The Associated Press in an interview Saturday that the bomb had been put together inside the Green Zone and had been intended to kill him when he was visiting parliament.

Ever since the blast, the Iraqi security forces that man the multiple entrances going into and out of the Green Zone have intensified their checks.

The U.S. Embassy on Saturday announced on its website that it was severely limiting the movements of American government employees inside the Green Zone. The embassy said it was taking the steps as a result of "severe threats of kidnapping operations and terrorist attacks throughout Iraq, including the International Zone," as they usually refer to the Green Zone.