US troops were sent to Syria this summer to try to rescue a number of kidnapped Americans, including journalist James Foley, but they did not find the hostages, senior White House officials said on Wednesday.

The officials said the rescue mission was authorized by President Barack Obama after intelligence agencies believed they had identified where the hostages were being held.

But the several dozen special operations troops who were dropped by aircraft into Syria did not find them and engaged in a firefight with Islamic State group militants before departing.

The officials said a number of militants, but no Americans, were killed.

One American sustained a minor injury when an aircraft was hit, according to the officials, speaking a day after a video showing the beheading of Foley was released by Isis militants.

The kidnappers also threatened to kill a second hostage, Steven Sotloff, if US airstrikes against the militants in Iraq continued.

The officials would not say specifically when or where the operation took place, citing the need to protect operational details in order to preserve the ability to carry out future rescue missions.

AP