A suicide bomber targeting a group of pilgrims heading to one of Shi'ite Islam's holiest festivals killed 25 people and wounded 45 others south of Baghdad on Sunday, Iraqi police said.

The bomber struck in the town of Iskandariya hours after militants killed three pilgrims and wounded 36 others in an attack in Baghdad, police said. Some reports put the toll at 40.

In the Baghdad attack, the pilgrims were hit by a roadside bomb and then fired on by gunmen on a road used by thousands of pilgrims walking to the festival of Arbain in the holy southern Shi'ite city of Kerbala.

Millions of Shi'ite pilgrims are expected in Kerbala for the festival this week, which commemorates the end of the 40-day mourning period following Ashura, a religious ritual that marks the death of Prophet Mohammad's grandson in 680.

Many Arbain pilgrims prefer to walk to Kerbala, 110 km (70 miles) south of Baghdad, because they believe the effort will bring them greater spiritual reward.

Security has been tightened compared to previous years, Kerbala's police chief Major-General Raad Shakir told Reuters last week, and Iraqi tanks are being used to protect the city for the first time, in addition to 40,000 police and soldiers.

All public transport, including bicycles, has been banned within a 25 km (15.5 mile) radius of the city, and 600 female security staff have been assigned to search women, police said.

Militants have used horses and carts, bicycles and motorcycles in bomb attacks in the past. There has also been a spate of suicide bombings carried out by women in recent months.

In previous years, militants have killed scores of pilgrims in suicide bombings and other attacks. Sunni Islamist Al Qaeda views Shi'ites, a majority in Iraq but a minority in the Muslim world, as heretics.

Suspected Sunni Arab insurgents killed 149 pilgrims on their way to Kerbala for Arbain last year in one of the worst spasms of violence since the US-led invasion of Iraq in 2003.

Last August, clashes between rival Shi'ite factions during another religious festival in Kerbala killed dozens of people and forced the hurried evacuation of hundreds of thousands of pilgrims. -Reuters