President Barack Obama on Friday strongly condemned the "brutal murder" of a British hostage by the Islamic State group.

An Internet video released Friday purports to show an Islamic State fighter beheading Alan Henning, the fourth such killing carried out by the extremist group now targeted in U.S.-led airstrikes.

"Mr. Henning worked to help improve the lives of the Syrian people and his death is a great loss for them, for his family and the people of the United Kingdom," Obama said in a statement.

"Standing together with our U.K. friends and allies, we will work to bring the perpetrators of Alan's murder - as well as the murders of Jim Foley, Steven Sotloff and David Haines - to justice," Obama said.

The president said that the U.S. and other nations will continue to take action to "degrade and ultimately destroy" the Islamic State group.

The National Security Council, which advises the president, said it was using all means to bring home another U.S. citizen held by the extremists, Peter Kassig, a former Army Ranger from Indianapolis. In a separate video released Friday, Kassig is shown kneeling while a masked militant says Kassig will be killed next.

"We can confirm that U.S. citizen Peter Kassig is being held by ISIL," NSC spokeswoman Caitlin Hayden said, referring to an Islamic State acronym. At this point we have no reason to doubt the authenticity of the video released earlier today. We will continue to use every tool at our disposal -- military, diplomatic, law enforcement and intelligence - to try to bring Peter home to his family."