The role of Australia’s defence force in helping Kurdish forces in Iraq has been spelt out by the prime minister in a speech to parliament.

Prime Minister Tony Abbott understands apprehension about Australia again getting involved in an Iraq conflict but says doing nothing also has its risks.

Mr Abbott on Monday used a speech to parliament to spell out the government’s reasons for taking part in an air drop of weapons and aid to Kurdish fighters taking on Islamic State extremists in northern Iraq.

“Many Australians are understandably apprehensive about the risk of becoming involved in another long and costly conflict in the Middle East,” Mr Abbott said.

“Doing anything involves serious risks and weighty consequences. But doing nothing involves risks and consequences too.”

He said he would no longer be referring to the extremists as Islamic State.

“It is not a state, it’s a death cult,” Mr Abbott said.

“Australia cannot leave the Iraqi people to face this horror, this pure evil, alone.”

Mr Abbott said if the Obama administration and the Iraqi government requested military support it would be considered against four criteria.

These included a clear and achievable overall objective, a proportionate role for Australian forces, a proper risk assessment and an overall humanitarian objective in accordance with Australia’s national interests.

“Like President Obama, Australia has no intention to commit combat troops on the ground, but we’re not inclined to stand by in the face of preventable genocide either.”

He said there was a risk that Australians involved in the conflict could bring their skills home with deadly consequences.

Labor and the coalition earlier rejected an Australian Greens bid for a parliamentary vote to approve Australian action in Iraq.

Mr Abbott said speeches in response to his statement should suffice for a debate.

Opposition Leader Bill Shorten said Labor’s support for the government was underpinned by three key principles – effective response to the growing humanitarian crisis, promoting a unity government in Iraq and dealing with Australian foreign fighters.

“We should not confuse empty jingoism and aggressive nationalism with steady decision-making. Neither can we ignore the dreadful consequences of fanaticism and extremism,” he said.

Mr Shorten said Islamic State was an enemy of humanity engaged in crimes against humanity.

“We cannot co-operate with this evil by refusing to support the innocent,” he said.