"We have seen these types of words from ISIL before," State Department spokeswoman Jen Psaki told reporters, referring to militants of the Islamic State of Iraq and the Levant.
"This declaration has no meaning for the people in Iraq and Syria," she said, adding that the militants -- who have now renamed their group the Islamic State -- were just trying "to control people by fear."
The White House said the jihadists spearheading the offensive in Iraq had waged "a campaign of terror of gross acts of violence and repressive ideology that pose a grave threat to Iraq's future."
With Iraq's parliament scheduled to hold its opening session tomorrow, Psaki said Washington was "continuing to urge Iraqi leaders to come to an agreement on the critical posts that are key" for the formation of a new government.
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The jihadists, in an audio recording released online yesterday, said their caliphate would spread from Aleppo in northern Syria to Diyala in eastern Iraq, and ordered Muslims in those areas to "obey" and pay allegiance to their new leader.
After the first four caliphs who succeeded Mohammed, the caliphate lived its golden age in the Omayyad empire from the year 661 to 750, and then under the Abbasids, from 750 to 1517.
It was abolished in 1924 after the Ottoman empire collapsed.