President Barack Obama will make a statement shortly after noon Wednesday as the White House said the US launched fresh Iraq air strikes despite militants' threat to execute a second US journalist.

Obama will speak at 12:45 pm (1645 GMT), a day after Americans were horrified by the beheading of US journalist James Foley by Islamic State jihadists.

The White House confirmed that the video of Foley's killing, released on the Internet Tuesday, was authentic after US justice and security officials studied it.

The nearly five-minute video, apparently filmed in the Iraq or Syrian desert, also showed a second American journalist held by the group, Steven Sotloff.

In the video, titled "A Message to America" and distributed online by known Islamic State sources, the group declares that Foley was killed after Obama ordered air strikes against IS positions in northern Iraq.

The group threatened to kill Sotloff, saying whether it does or not depends on Obama's actions.

Pentagon spokesman Rear Admiral John Kirby said the air strikes in Iraq, which have targeted IS militants for more than a week, have continued.

"We continue to conduct strikes in Iraq," he said.

"Without getting into details I can tell you that there have been additional strikes in Iraq, they're tied to the authorization through which we are conducting strikes."

The official said the air strikes were conducted in northern Iraq where a US air campaign has targeted Islamic State forces threatening civilian refugees and the Kurdish regional capital Arbil.

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