WASHINGTON -- The murderers of James Foley and Steven Sotloff taking refuge in Syria will be held accountable by the United States, "no matter how long it takes," US Secretary of State John Kerry on Wednesday, mourning the beheading of a second American in two weeks at the hands of Islamic State.

Both Kerry and US President Barack Obama suggested that the scope of America's mission against Islamic State, a terrorist Islamist group, has been broadened. The fundamentalist army has conquered territory throughout northern Iraq, but has held court in Syria's eastern provinces for over a year.

Sotloff, 31, an Israeli-American journalist, was killed by Islamic State militants seeking retribution against the US for its air campaign against their assets throughout northern Iraq. It was not immediately clear whether Sotloff's captors knew of his religion, or second nationality.

A video showing his killing was authenticated by the US intelligence community overnight.

"Our objective is clear, and that is to degrade and destroy ISIL so that it’s no longer a threat not just to Iraq but also the region and to the United States," Obama told press during a visit to Estonia. "Those who make the mistake of harming Americans will learn that we will not forget, and that our reach is long and that justice will be served."

Yet the president also said that a strike against Islamic State targets in Syria "might require congressional approval," far from guaranteed.

In the video, Sotloff's killer, a masked man believed to be British and to have also killed Foley in August, threatens to next kill David Cawthorne Haines, a Briton, should US air strikes continue.

British Prime Minister David Cameron responded angrily to the developments, threatening to join the US air campaign.

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