Washington - The murder of US journalist James Foley by the so-called Islamic State is the extremist group's "first terrorist attack against the United States", a former head of the CIA warned on Wednesday.
Michael Morell, a former deputy director of the intelligence agency who was its acting chief between 2012 and 2013, said the jihadist group is trying to intimidate Washington into calling off air strikes.
"And I think our response should be and our response will be to not do that. In fact, we should pick up the pace here," he told CBS News, referring to recent US strikes against Islamic State militants in Iraq.
"The definition of terrorism is political violence, violence for political effect. So we should mark this date down because this is ISIS's first terrorist attack against the United States."
The Islamic State, formerly known as the Islamic State in Iraq and the Levant (ISIS), declared itself a "caliphate" in late June and has since added a swath of northern Iraq to territory it holds in eastern Syria.
Earlier this month, US President Barack Obama ordered US warplanes to strike Islamic State targets in Iraq to defend the Kurdish regional capital Arbil and civilian refugees fleeing the jihadist advance.
This week, with assistance from US bombing runs, Kurdish and Iraqi forces began to push Islamic State forces back and recaptured a strategic dam.
‘Merciless murder’
German Chancellor Angela Merkel is "horrified" about the fate of Foley, her spokesperson said on Wednesday.
The spokesperson, Steffen Seibert, told reporters that the video released by Islamic State jihadists depicted "the barbaric, truly merciless murder of a person", calling it a "repulsive document".
"It shows that this terror group has nothing to offer but cruelty and fanaticism," Seibert said.
"The chancellor is horrified about the fate of the American journalist James Foley, who was violently abducted in 2012, although there is no official confirmation that it is his murder shown in the video."
Seibert said the German government expressed its "deepest sympathies" to Foley's family for their nearly two-year ordeal "and for the suffering that this video surely inflicted".
On Tuesday the group released a video showing the execution-style murder of Foley, a 40-year-old US reporter who was captured in Syria in late 2012 and held hostage ever since.
In addition to rallying extremist elements of the Sunni insurgencies against the governments of Iraq and Syria, Islamic State has recruited radical jihadists from around the world, including Western countries.
Foley's killer spoke with a British accent in the
released footage, increasing fears that the group may one day mobilise foreign
fighters to strike targets in Europe and America.