The White House has condemned the beheading of Egyptian Christians in Libya, calling the act "despicable and cowardly murder."
In a video released by ISIS' Al-Hayat Media Centre, the beheaded Christians were seen dressed in orange jumpsuits. Each of the victims was paired with an ISIS member holding knives in their hands. They were forced down on the sand before being beheaded in a brutal mass execution that has been condemned by world leaders.
The recording was subsequently posted to the web with the title "A Message signed with blood to the nation of the cross."
Hours after the video's release, Egypt said it responded by bombing Islamic State targets in Libya.
The US was among the countries condemning the beheadings. "ISIL's barbarity knows no bounds. It is unconstrained by faith, sect, or ethnicity," the White House said in a statement.
The beheading of the 21 Christians is a "wanton killing of innocents" and this most recent display of violence by the ISIS, including the murder of many Egyptian soldiers in the Sinai, only unites the international community in its fight against the terrorist group, the statement said.
"We call on Libyans to strongly reject this and all acts of terrorism and to unite in the face of this shared and growing threat," said White House press secretary Josh Earnest.
"We continue to strongly support the efforts of the United Nations Special Representative of the Secretary-General Bernardino Leon to facilitate formation of a national unity government and help foster a political solution in Libya."
The US government offered its condolences to the families of the beheaded Christians and has vowed support to the Egyptian government as it faces off the threat of ISIS.
Global security firm Flashpoint Intelligence told NBC News that the latest beheadings by ISIS suggests a specific intent to attack Christians.
"This undeniably means that the group now views Christian populations as not only targets but also part of the bigger 'Crusader plot,' not separate from the US-led coalition or aggressors," a Flashpoint report said. "The group's message is highly intimidating and it somewhat challenges the Western nations to intervene and save the Christians as it intervened to save the Yazidis and others."