Islamic State of Iraq and Syria (ISIS) militants have beheaded Peter Kassig, the captured humanitarian worker, U.S. President Barack Obama announced Sunday.

A report by Time revealed that 26-year-old Kassig had served U.S. Army Ranger before he worked as a medical aid in Syria. He was held captive by the ISIS group October last year, the Associated Press reports.

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It was revealed that Kassig converted into Islam, taking the name Abdul-Rahman. Kassig was the third American citizen to be murdered by the Islamic militant group.

In a statement, President Obama condemned ISIS for doing the "act of pure evil by a terrorist group that the world rightly associates with inhumanity."

Meanwhile, he praised Kassig for his selfless endeavors, while helping innocent civilians at the height of the civil was in Syria, to the point of putting his life at risk and death. 

Kassig's parents, Ed and Paula, said in a statement that their son's execution was heart-wrenching but they still believe that their son's life did not go for nothing. It was the outcome of his love for the Syrian citizens and his efforts to alleviate their suffering, Kassig's parents added.

"We are incredibly proud of our son for living his life according to his humanitarian calling," the couple said. 

The execution was captured in a 16-minute video, which took place in Dabiq, a town in Syria.

Several Syrian soldiers were also murdered.

The video was posted on websites used by the group. The ISIS' killings of U.S. citizens serve as a warning to stop its airstrikes in Iraq and Syria.

The U.S. launched the armed campaign against said Muslim militants earlier this year to stop their expansion.

Meanwhile, U.S. Secretary of State John Kerry compared Kassig and the ISIS. "Abdul-Rahman's generosity of spirit" he said, greatly contrasted "the pernicious evil" that is ISIS.

Kerry narrated how Kassig's family and the U.S. government worked hard to prevent the tragic outcome from happening. The victim's mother even pleaded for his son's life but the request went unheeded.

Kassig's murder - as well as the death of other hostages - only served to fuel the hatred that the Western world has against ISIS, even building support for the airstrike against the ISIS, which claims that it's members are already growing in Middle East and in other parts of the world. 

TagsPeter Kassig, ISIS, ISIL, ISIS video, beheading, American aid worker