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.


"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.