Obama: Islamic state "stand for no religion"
Obama's response on Wednesday to the execution of James Foley marked his strongest condemnation yet of IS fighters, and he gave no sign of a pause in US targeting of its positions in Iraq.
The president's remarks to reporters covering his vacation in Martha's Vineyard, Massachusetts, came shortly after the White House announced that a video showing the beheading of Foley had been authenticated by the US intelligence community, the Reuters news agency reported.
Obama said he had called Foley's family to express his condolences.
"The United States of America will continue to do what we must do to protect our people," Obama said.
"We will be vigilant and we will be relentless. When people harm Americans, anywhere, we do what's necessary to see that justice is done."
Obama said the fighters had rampaged across cities and villages, abducted women and children and subjected them to torture and rape and killed Muslims, both Sunni and Shia, by the thousands.
"No just God would stand for what they did yesterday and what they do every single day," he said. "Their ideology is bankrupt."
US Secretary of State John Kerry later called the Islamic State group an "Ugly, savage, inexplicable, nihilistic, and valueless evil."
"ISIL is the face of that evil, a threat to people who want to live in peace, and an ugly insult to the peaceful religion they violate every day with their barbarity," he added.