Pressure on Western governments to take firm action against the jihadists in Iraq and Syria has risen sharply after the brutal videotaped execution of a second American journalist and a threat to kill a captive British aid worker.
The beheading of 31-year-old reporter Steven Sotloff came amid mounting evidence from the United Nations and human rights groups of the scale of the atrocities committed by IS fighters in northern Iraq and eastern Syria against ethnic and religious minorities.
"And the question is going to be making sure we've got the right strategy, but also making sure that we've got the international will to do it," he said in Estonia before heading to the summit in Wales.
With one of its nationals also under threat of beheading, summit hosts Britain said it would not rule out taking part in air strikes if necessary.
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"I can assure you that we will look at every possible option to protect this person," Foreign Secretary Philip Hammond said.
"The head of state underlined the importance of a political, humanitarian and if necessary military response in accordance with international law," his office said.
Obama pledged that justice would be done to the killers of 31-year-old reporter Steven Sotloff, wherever they hid and however long it took.
Obama will lead a UN Security Council session on the threat of foreign fighters in Iraq and Syria on September 25, a US official said yesterday.
His Secretary of State John Kerry revealed he was working to forge a global coalition to fight the "mediaeval savagery" of Islamic militants terrorising a swathe of Syria and Iraq.