The IS released a video Saturday which showed the beheading of British aid worker David Haines, who was kidnapped last year in Syria, Xinhua reported.
A masked IS militant on the video claimed the execution was in retaliation for British Prime Minister David Cameron's promise to join a US-led coalition to fight the IS.
"The United States strongly condemns the barbaric murder of UK citizen David Haines by the terrorist group ISIL. Our hearts go out to the family of Mr. Haines and to the people of the United Kingdom," Obama said in a statement issued by the White House.
"The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve. We will work with the United Kingdom and a broad coalition of nations from the region and around the world to bring the perpetrators of this outrageous act to justice, and to degrade and destroy this threat to the people of our countries, the region and the world," Obama added.
The video was similar to those of the recent executions of two American journalists, James Foley and Steven Sotloff, which were also posted online.
At the end of the latest video, the masked IS militant, who appeared to be the same killer in the previous two videos, threatened to kill another British hostage named Alan Henning, if Cameron continued to support the fight against the IS.
Cameron earlier condemned the killing of Haines as "a despicable and appalling murder" and "an act of pure evil" by the IS. "We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes," he vowed.