A roadside bomb has killed three people in central Iraq, including the son of a tribal leader.

Naeim al-Halbosi, a leader of a Sunni tribal group opposed to al-Qaida, was injured in Saturday's blast near Fallujah.  His son and two bodyguards were killed.

Meanwhile, hundreds of thousands of pilgrims are streaming into Baghdad to visit an important Shi'ite holy site.

The Kazimiyah shrine also has been the frequent target of Sunni militant attacks.  

The pilgrimage marks one of the first major security challenges Iraqi forces have faced on their own since U.S. troops withdrew from Iraqi cities.

In another development, a security helicopter used to protect diplomats in Iraq crashed just outside of Baghdad, killing two crew members.

An investigation is underway to determine the cause of the crash.

Officials say the MC-530 chopper was on a training exercise when it went down.

The United States contracted the security firm that owns the helicopter.  The company, Xe, was formerly known as Blackwater.

The firm recently lost its license to work in Iraq, after employees working for the company when it was known as Blackwater were involved in a controversial shooting that killed at least 14 civilians in 2007.

In May, a spokeswoman at the U.S. Embassy in Baghdad said the security firm had finished its work in the Iraqi capital but said it still had contracts in other areas of Iraq that were set to end soon.

Some information for this report was provided by AFP, AP and Reuters.