U.S. Vice President Joe Biden telephoned Iraqi Kurdistan President Masoud Barzani on Saturday to express condolences for the beheadings of three Kurdish fighters by Islamic State last week.
The White House said both agreed that "such unconscionable acts of brutality reflect ISIL's [Islamic State's] true nature and reinforce our collective resolve to defeat ISIL."
Barzani promised to avenge the deaths. He wrote in an online statement, "Those who martyr the peshmerga [Kurdish fighters] in this way before, now, and in the future will see how the hand of the heroic peshmerga will reach them."
Islamic State on Friday posted an Internet video showing people it said had been wounded by Kurdish rocket fire. It then shows three Kurdish fighters being beheaded, one at a time.
A militant dressed in black, addressing Barzani, threatened more executions if shelling continued.
In neighboring Syria, human rights monitors blamed Islamic State for two attacks on Kurds celebrating the Persian new year that killed 45 people.
The British-based Syrian Observatory for Human Rights said a suicide bomber and a car bomb were aimed at Kurds in the northeastern city of Hasakah. The blasts wounded more than 120 people.
U.N. Secretary-General Ban Ki-moon condemned what he called the "heinous attacks." He said they came "during a holiday that is customarily a time for Kurdish communities to come together to share their hopes for the new year."
Ban also responded to reports linking Islamic State to the attacks.
"The terrorist organization's despicable agenda includes efforts to incite sectarian and ethnic division among Syria's diverse communities," he said. "It must not be allowed to succeed."
Syria's state-run news agency said the attacks were carried out by "terrorists" — its word for rebels fighting to overthrow President Bashar al-Assad.
The commander of Kurdish security forces in Syria said Friday's bombings "will not pass without retribution."