Shiite Muslims from around the world thronged an Iraqi shrine city on Thursday amid heavy security after attacks killed 17 worshippers during annual commemoration rituals often targeted by Sunni militants.

The Ashura ceremonies, which mark the death of a venerated figure in Shiite Islam, come amid a months-long surge in violence nationwide that has forced Baghdad to appeal for help from Washington in combating militancy.

The bloodshed over the two days that mark the peak of Ashura struck across Iraq, with separate bombings targeting gatherings of Shiites in a restive province north of Baghdad, and a procession tent south of the capital.

Twin bombings in the town of Hafriyah, where Shiites were gathered at a tent, killed at least nine people and wounded dozens of others early on Thursday, while the previous day, eight people were killed in three coordinated blasts targeting another group of worshippers, officials said.

A car bomb also targeted a Shiite man distributing food in the northern city of Kirkuk, leaving eight wounded on Wednesday.

Shiites from Iraq and around the world mark Ashura, which this year climaxes on Thursday, by setting up procession tents where food is distributed to passers-by and pilgrims can gather.

Some two million will make the pilgrimage, often on foot, to the Iraqi city of Karbala, which is home to a shrine to Imam Hussein.

Hussein, grandson of the Muslim Prophet Mohammed, was killed by the armies of the caliph Yazid in 680 AD and his death in Karbala has come to symbolise the split between Islam's Sunni and Shiite sects.

Tradition holds that the venerated imam was decapitated and his body mutilated.

To mark the occasion, modern-day Shiite mourners flood Hussein's shrine, demonstrating their ritual guilt and remorse for not defending him by beating their heads and chests and, in some cases, making incisions on their scalps with swords in rituals of self-flagellation.

Pilgrims throng shrines

On Thursday, black-clad pilgrims packed the shrines of Hussein and his half-brother Abbas, listening over loudspeakers to the story of the battle in which Hussein was killed as volunteers distributed food and water.

They will later carry out a ritual run to the shrine, striking their heads and shouting, "We sacrifice for you, O Hussein!"

"I have been coming since I was young, every year, even during the time of the tyrant Saddam," said Abu Ali, or father of Ali, a 35-year-old pilgrim visiting from the southern port city of Basra, referring to the rule of now-executed dictator Saddam Hussein.

His Sunni-dominated regime barred the vast majority of Ashura commemorations, as well as the subsequent rituals of Arbaeen.

"I challenge anyone not to cry," the worshipper said, describing his emotions upon taking part in Ashura rituals.

The commemorations, which also include a reenactment of the attack that killed Hussein, were due to wrap up after the run, which takes place following midday prayers.

Provincial authorities expect two million pilgrims, including 200,000 from outside Iraq, will have visited Karbala in the 10 days leading up to Ashura, with all of the city's hotels fully booked.

Shiites make up about 15 percent of Muslims worldwide. They are a majority in Iraq, Iran and Bahrain, and there are large Shiite communities in Afghanistan, Lebanon, Pakistan and Saudi Arabia, and Syria.

Sunni militants linked to Al-Qaeda, who regard Shiites as apostates, often step up their targeting of Iraq's majority community during Ashura and Arbaeen, including by attacking pilgrims.

Security measures have been stepped up, with more than 35,000 soldiers and policemen currently deployed to Karbala and surrounding areas, with concentric security perimeters barring vehicles from entering the city while helicopters hover overhead.

The violence against Shiites is the latest in Iraq's worst unrest since 2008 and forced Prime Minister Nuri al-Maliki to appeal to the United States for help in the form of intelligence sharing and the delivery of new weapons systems in an effort to deal with the unrest.

Turkey's foreign minister also offered Ankara's assistance during a recent visit to Baghdad.