"We've seen the reports of the video. We have no reason to doubt its authenticity," State Department Spokesperson Jen Psaki told reporters at a news conference.
The Sunni militant group has seized large areas of northern and western Iraq in an offencive that began last month.
Psaki said that the US was "encouraging urgent steps toward a government formation (in Iraq), and we have a range of options at our disposal to take on the threat that Iraq and the region is facing from ISIL (Islamic State of Iraq and the Levant)."
Expressing concern about the adjournment of Iraq's next parliamentary session to mid-August, Canadian Foreign Minister John Baird, in a statement, said that any postponement of the parliamentary process would further undermine the stability, security and unity of violence-ravaged country.
"The Iraqi people are in need and deserving of unified leadership at this critical time," Baird said.