"We are all outraged at reports from Iraq about the brutal killing of civilians by ISIL (IS), including yesterday's reported brutal beheading of another journalist," he said in New Zealand after a video purporting to show the death of US journalist Steven Sotloff was posted online.
The release of the images follows similar grisly footage from IS last month showing another US journalist, James Foley, being decapitated.
"I strongly condemn all such despicable crimes and I refuse to accept that whole communities can be threatened by atrocity crimes because of who they are or what they believe," Ban said.
In the latest footage, Sotloff, 31, calmly addresses the camera to say he is a victim of US President Barack Obama's decision to press on with air strikes in Iraq against the jihadists.
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A masked militant with a British accent, possibly the same man shown killing Foley in the first video, then murders him with a knife.
Sotloff, a veteran war reporter well versed in the history and culture of the Middle East, was captured just over a year ago while crossing the frontier from Turkey into Syria.
The UN Human Rights Council this week unanimously agreed to send an emergency mission to Iraq to investigate atrocities in the self-declared Islamic "caliphate" amid reports of ethnic cleansing in minority Christian and Yazidi Kurdish areas.
The United States has conducted air strikes in Iraq in a bid to limit IS gains and protect ethnic minorities, although it has so far ruled out cooperating with the Damascus regime on similar action in Syria.
Germany has agreed to send arms to Iraqi Kurds while Australia is helping transport weapons to the Kurdish fighters and has carried out humanitarian air drops to besieged communities.