* Islamic State of Iraq is al Qaeda umbrella group
* Attacks on security agency, Asiacell killed 12 people
BAGHDAD, Sept 24 (Reuters) - Al Qaeda's Iraqi affiliate claimed responsibility for a double car bomb attack this week that killed at least 12 people in Baghdad, a group that monitors insurgent communications said on Friday.
On Sept 19, one blast targeting a national security office killed at least six people and wounded 15, damaging nearby houses. A second car bomb five minutes later near an office of mobile phone operator Asiacell, part-owned by Qatar Telecommunications Co <QTEL.QA>, killed six and wounded 51 people. [ID:nLDE68I02Z]
The Islamic State of Iraq, an umbrella group for al Qaeda-linked insurgents, claimed responsibility for both attacks in a statement posted on a radical Islamic website, the U.S.-based SITE Intelligence Group said.
The group said it "managed to break through the heavy barriers of the army and the apostate police that had been placed in the two fortified neighbourhoods, and planted two booby-trapped cars near the two targets," the ISI statement said, according to SITE.
ISI accused Asiacell of being an "inseparable part of the security organisation" that pursues and spies on the mujahideen, according to the statement.
Separately, Shi'ite militia Asaib al-Haq, or Leagues of Righteousness, vowed to stage attacks on U.S. forces in Iraq, according to a statement obtained by Reuters on Friday.
Asaib al-Haq splintered from the Mehdi Army of anti-American Shi'ite cleric Moqtada al-Sadr.
Overall violence has fallen sharply since the height in 2006/07 of the sectarian warfare unleashed after the 2003 U.S.-led invasion. But attacks by a Sunni Islamist insurgency are still a daily occurrence.
The end of the U.S. combat mission on Aug. 31 has raised fears of a return to broader bloodshed.
Insurgents are trying to exploit a political vacuum created by a failure of Sunni, Shi'ite and Kurdish factions to agree on a new government more than six months after a March 7 election.
(Reporting by Khalid al-Ansary and Muhanad Mohammed; Editing by Rania El Gamal and Peter Graff)
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