A roadside bomb blew up near a bus carrying Iranian pilgrims in northern Baghdad on Wednesday, killing at least two people and wounding 18 others, security and hospital sources said.

The bus was attacked as the pilgrims returned from a visit to a Shi'ite Muslim shrine in the city of Samarra, 100 km north of the Iraqi capital.

Iraqi security forces are trying to quell a stubborn Sunni Muslim insurgency more than eight years after the U.S.-led invasion that ousted Sunni dictator Saddam Hussein.

Military leaders have expressed concern that militants will step up attacks as U.S. forces withdraw. The remaining 24,000 U.S. troops are due to leave before year-end under a 2008 security pact between the two countries.

Insurgents have frequently targeted Iranian pilgrims, who have flocked to Shi'ite holy sites in Iraq since the 2003 invasion.

On Sunday, a roadside bomb blew up near a bus in the northwestern Baghdad district of Kadhimiya, killing one Iranian pilgrim and wounding four others, according to a police source.

(Reporting by Kareem Raheem; Writing by Jim Loney; Editing by Mark Heinrich)