President Barack Obama condemned as a "barbaric murder" the slaying of British aid worker David Haines by Islamic State extremists.
Haines was abducted last year in Syria, and extremists released a video on Saturday showing his beheading.
In a statement issued Saturday night, Obama said the hearts of Americans go out to Haines' family and the people of the United Kingdom.
"The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve," Obama said.
"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," he said.
In his statement, Obama repeated the pledge he made Wednesday night in a nationally televised address in which he laid out a strategy to respond to the threat from the Islamic State group, which is referred to at times as ISIL. "Our objective is clear," he told the nation earlier this week. "We will degrade, and ultimately destroy, ISIL through a comprehensive and sustained counter-terrorism strategy."
The U.S. began attacking the extremist group with airstrikes in Iraq last month. Since then, the extremists have beheaded two American journalists held captive in Syria, James Foley and Steven Sotloff.
As part of the effort against the Islamic State group, Secretary of State John Kerry formally announced Saturday that retired Marine Gen. John Allen was joining the State Department as a special presidential envoy to coordinate the international coalition. Allen worked with international allies as the top U.S. commander in Afghanistan from 2011-2013.