Department spokesman Mark Toner said Kerry is taking a "measured" approach and while his decision will come "soon" it will not meet today's deadline. Other officials said they expected the determination could be made next week.
"He recognises the seriousness of the atrocities committed by this terrorist group as well as the importance of this issue to its victims and survivors," Toner told reporters.
Congress had set a March 17 deadline for the determination. Earlier this week, the House passed a non-binding resolution by a vote of 393-0 condemning Islamic State group actions as genocide.
Reaction to the delay from lawmakers was swift. "This is heartbreaking. There has been ample time for analysis. The evidence of ISIS genocide against Christians, Yezidis, and others is horrifyingly clear," said Rep Jeff Fortenberry, who authored the bill. "I cannot understand the hesitation by the State Department."
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An executive branch determination of genocide by the Islamic State would mark only the second time a U.S. administration has reached that conclusion while a conflict is ongoing.
The first was in 2004, when Secretary of State Colin Powell determined that atrocities being committed in Sudan's Darfur region constituted genocide.
Powell reached that determination amid much lobbying from human rights groups but only after State Department lawyers advised him that it would not contrary to legal advice offered to previous administrations - obligate the United States to take action to stop it.
Powell instead called for the UN Security Council to appoint a commission to investigate and take appropriate legal action if it agreed with the genocide determination.
Kerry faces similar issues, although Toner suggested Kerry has already received a similar legal opinion.