Left unchecked Isis would "rain carnage on the world", Prime Minister John Key says, but he has yet to make a decision on whether New Zealand troops will join a coalition fight against the extremist group.
Speaking to Radio New Zealand this morning, Mr Key said he was still weighing up the risks and benefits of New Zealand adopting a military response against Isis, vowing to "carefully trod our way through this".
However, he was of the belief that Isis were "very bad people" and a military response was morally justified.
"They're very bad people and left unchecked they will rain carnage on the world, that's my view of these people," he told the broadcaster.
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"And they're somewhat out of control.
"[But] there are many situations around the world where there are real issues, the question is how we can best contain [them] and potentially ensure that they desist from their activities."
Pushed on whether a military response was morally right, Mr Key said: "On balance I would say yes, but that doesn't mean that we would actually do that.
"If you go and look in Iraq, what we see in Iraq is basically a situation where, a, the Iraqi government in asking for help, [so] you're not doing something against the will of the people, and secondly, the actions that these people are undertaking - everything from public beheadings to the actions against innocent civilians - then it's morally reprehensible.
"I think the question is would a military response from New Zealand play both a useful part and be worth what we're doing."
He was likely to make a decision "within the next few weeks", he said.