The release of the video comes a day after Afghanistan's president blamed the extremists for a suicide attack in his country that killed at least 35 people - and underscores the chaos gripping Libya after its 2011 civil war and the killing of dictator Moammar Gadhafi.

Ethiopia long has drawn the anger of Islamic extremists over its military's attacks on neighboring Somalia, whose population is almost entirely Muslim. While the militant in the video at one point said "Muslim blood that was shed under the hands of your religion is not cheap," it did not specifically mention the Ethiopian government's actions.

"If this is confirmed, it will be a warning to people who wish to risk and travel to Europe though the dangerous route," Hussein said.

"I believe this is just another case of the ISIS group killing Christians in the name of Islam. Our fellow citizens have just been killed on a faith-based violence that is totally unacceptable. This is outrageous," Mulugeta said. "No religion orders the killing of other people, even people from another religion."

"The Islamic State in Libya is still focused on this consolidation phase of announcing its presence through these very high-profile executions," said Frederic Wehrey, a senior associate for the Middle East Program at the Carnegie Endowment for International Peace. "But they face some structural limits in terms of how much local support they can get because they haven't captured real revenue streams."

"Our top priority is delivering life-saving assistance to people who are fleeing - food, water and shelter are highest on the list of priorities," said Lise Grande, the U.N. humanitarian coordinator for Iraq. "Seeing people carrying what little they can and rushing for safety is heart-breaking."

 Iraqi troops backed by Shiite militias and U.S.-led airstrikes managed to dislodge the Islamic State group from the northern city of Tikrit earlier this month. But the troops have struggled against the militants in Anbar, which saw some of the heaviest fighting of the eight-year U.S. military occupation that ended in 2011.