The Islamic State is expanding beyond its Syrian and Iraqi base by bringing local Islamist groups under its wing, International Business Times reported.

In the latest example of this strategy, the Islamic State (ISIS) released a video showing 21 Egyptian Coptic Christians taken hostage in Libya some weeks ago being beheaded by one of its satellite groups.

The killings bring home the continued expansion of ISIS into chaotic Libya, where a civil unrest plagues the government.

ISIS has similarly penetrated into the Egyptian Sinai peninsula, and claimed inroads in Yemen, Saudi Arabia, and Algeria by capturing the allegiance of local jihadist groups to its so-called caliph, Abu Bakr al-Baghdadi, the Business Times reported.

Libya has been vulnerable to Islamist extremist groups since the fall of Moammar Gadhafi in 2011. Some groups, like the Shura Council of the Youth of Islam, broke with al-Qaida and have aligned with ISIS. Others, such as Ansar al-Sharia, continue to maintain ties with al-Qaida while not criticizing ISIS, according to the Business Times.

In Libya, ISIS has made inroads in Benghazi, Sirte, and Tripoli, and across southern parts of the country, according to Washington Institute for Near East policy analyst Andrew Engel.

Typically, when ISIS takes control of territory it sets up an Islamic governing council to impose a harsh version of Sharia law on locals. In Barqa, which is in eastern Libya, the group forbids the use of tobacco, demolishes shrines it deems un-Islamic, and establishes a network of social services to win over the population, according to Engel.

The beheading video showed masked ISIS-affiliated terrorists dressed in black holding machetes while marching their prisoners, who were wearing orange jumpsuits, along the beach. The film was produced by Al Hayat, the media arm of ISIS.

As the Christians, who had gone to Libya in search of work, are being slaughtered, the lead executioner makes a rambling address in American-accented English. He implies that the killings are getting close to "Rome," and that they are in retaliation for the death of Osama bin Laden and the alleged kidnapping of a Christian woman who purportedly wanted to convert to Islam, The New York Times reported.

"It is one thing to fly the ISIS flag because a lot of guys are doing it," William McCants, a researcher at the Brookings Institution who studies Islamist militants, told The New York Times. "It is another thing to capture a bunch of Egyptian Copts and kill them and see it as some of part of a grand, final-days battle."

Egyptian warplanes on Monday bombed ISIS targets in Libya in retaliation for the beheadings, according to press reports.

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