WELLINGTON (AFP) - New Zealand will send troops to Iraq on a non-combat mission helping to boost the local military's capacity to fight the Islamic State in Iraq and Syria (ISIS) organisation, Prime Minister John Key said on Tuesday.

Key said about 140 troops would begin a "behind the wire" mission in May after a request from the Iraqi government for international help in increasing its military capability to battle the militants.

"We cannot, and should not, fight Iraq's battles for them - and actually Iraq doesn't want us to," he told parliament. "Our military can, however, play a part in building the capability and capacity of the Iraqi forces so they can fight ISIL themselves."

Key said New Zealand was part of a 62-nation coalition against ISIS, which is also known as ISIL and has captured swathes of territory across Iraq and Syria.

He described the group - infamous for beheading, stoning and burning alive its victims - as "barbaric", saying New Zealand would "stand up for what's right".

"Sending our forces to Iraq is not an easy decision but it is the right decision," he said.

He added that New Zealand troops would most likely work alongside their Australian counterparts at a military base in Taji, north of Baghdad.

Key said the initial deployment was for nine months and the mission would not extend beyond two years.