Baghdad ? An al-Qaida front organization claimed responsibility Saturday for a suicide bombing that killed more than 20 people – including three Marines – as the U.S. military stepped up pressure on extremists in northern Iraq.

The Islamic State of Iraq posted the claim on a militant Web site, saying the bomber blew himself up among a gathering of the “heads of apostasy” – a reference to U.S.-backed Sunni tribal leaders who were attending a meeting Thursday in Karmah, 20 miles west of Baghdad.

“They sold their souls to the American devil for a cheap price,” the statement said. “Therefore, the soldiers of the Islamic State of Iraq have launched an open war against them.”