US President Barack Obama on Wednesday demanded the world take action against the "cancer" of jihadist extremism in Iraq, after an American reporter was murdered by militants.
As US jets continued to strike jihadist targets despite a threat to kill a second reporter, Obama said: "When people harm Americans anywhere, we do what's necessary to see that justice is done."
US Central Command said 14 air strikes had been carried out on IS targets in Iraq in the 24 hours since the video was released.
Obama was speaking after the so-called "Islamic State," which has seized much of eastern Syria and northern Iraq, released a video showing a masked militant beheading US reporter James Foley.
The black-clad man said the 40-year-old freelance journalist, who was kidnapped in northern Syria in November 2012, had been killed to avenge American air strikes against his movement.
He paraded a second US reporter, Steven Sotloff, before the camera and said he too would die unless Obama changes course. Pentagon confirmed that airstrikes would continue.
"There has to be a common effort to extract this cancer so it does not spread. It has to be a clear rejection of these kind of nihilistic ideologies," Obama said.
"One thing we can all agree on is that a group like ISIL has no place in the 21st century," he said. He said the group speaks for no true religion, and threatens Muslims and non-Muslims alike.
US intelligence believes the video is genuine, and the British government held a crisis meeting to launch an investigation because Foley's executioner spoke English with a London accent.
The United Nations and Europe's top powers condemned the killing, and France warned the world faces the "most serious international situation" since 2001, the year of the September 11 attacks.
In a significant shift from its usual policy, Germany said it was ready to send weapons to support Iraqi Kurds against IS, while France vowed to hold a conference on the security of the region.
Germany's development aid minister, Gerd Mueller, meanwhile yesterday accused Qatar of financing the jihadist group Islamic State.
"A story like this always has a history," he said in an interview with public broadcaster ZDF. "Who is financing these troops? Hint: Qatar."
Foley was an experienced correspondent who had covered wars in Afghanistan and Libya before heading to Syria, contributing reports to GlobalPost, Agence France-Presse (AFP) and other outlets.
Shia militia, federal soldiers, Kurdish troops and Sunni Arab tribes have been battling IS for weeks in some areas but have been unable to clinch a decisive victory. A strengthened US air support may turn the tide in the 2-month long fighting.