Shortly after Harper's plane touched down in London, he acknowledged reports of a video from the Islamic State in Iraq and the Levant purporting to depict the death of U.S. journalist Steven Sotloff.
"Appalled to learn of the barbaric and unacceptable death of Steven Sotloff at the hands of ISIL," Harper tweeted. "Our thoughts and prayers are with his loved ones."
It's the second such video to be released by ISIL in as many weeks. Last month, a similar video appeared to show the beheading death of James Foley, a U.S. reporter kidnapped in Syria 18 months earlier.
Harper is in London for trade discussions before heading to Cardiff for a two-day NATO summit, where the agenda includes a proposal to organize a new military force to give the alliance a fast-reaction capability.
The meeting, which runs Thursday and Friday, will also look at the continuing crisis in Ukraine and the end of the NATO mission in Afghanistan. It comes as the U.S. and Britain lean on Canada to increase defence spending to meet NATO's benchmark of two per cent of gross domestic product.
Sotloff, 31, who freelanced for Time and Foreign Policy magazines, vanished in Syria in August 2013 and was not seen again until he appeared in a video released online last month that showed Foley's beheading. Dressed in an orange jumpsuit against the backdrop of an arid Syrian landscape, Sotloff was threatened in that video with death unless the U.S. stopped airstrikes on the group in Iraq.
In the video distributed Tuesday and titled "A Second Message to America," Sotloff appears in a similar jumpsuit before he is beheaded by a fighter with ISIL, the extremist group that has claimed wide swathes of territory across Syria and Iraq and declared itself a caliphate.
The organization threatened to kill another hostage, this one identified as a British citizen.
In Washington, State Department spokeswoman Jen Psaki said U.S. intelligence analysis will "work as quickly as possible" to determine if the video of the beheading is authentic.
"If the video is genuine, we are sickened by this brutal act, taking the life of another innocent American citizen," Psaki said. "Our hearts go out to the Sotloff family and we will provide more information as it becomes available."
Psaki said it's believed that "a few" Americans are believed to still be held by ISIL but would not give any specifics.
— With files from The Associated Press