The Archbishop of Canterbury has been speaking in the House of Lords as MPs prepare to vote on whether the UK should be involved in military action.
He said:'There is justification for the use of armed force on humanitarian grounds, to enable oppressed victims to find safe space.
"ISIL – and for that matter Boko Haram and others – have as their strategy to change the facts on the ground so as to render completely absurd any chance of helping the targets of their cruelty.
"It is clear from talking this week with Christian and other leaders across the region that they want support.
"The solidarity in the region is added to by the important statement from the Grand Imam of al-Azhar on Wednesday.
The Archbishop spoke after Methodist and Baptist leaders warned against military action earlier in the week.
In a joint statement, they said:"The UK government must treat with caution suggestions that Islamic State could be defeated primarily through military action.
"An ideology – even one as dangerous and perverse as that of IS – cannot be defeated by the use of weapons."
Quakers in Britain has also warned against joining the US in air strikes.
Spokesperson Marigold Bentley said: "Killing people will trap us into a bloody cycle again.
"Can we not enter that bloody cycle and try a different way of dealing with this problem?"
Archbishop Justin did add that military action mustn't be the only option for dealing with Islamic State militants.
He said: "The action proposed today is right, but we must not rely on a short-term solution on a narrow front to a global, ideological, religious, holistic and trans-generational challenge.
"We must demonstrate that there is a positive vision far greater and more compelling than the evil of ISIL and its global clones. Such a vision offers us and the world hope, an assurance of success in this struggle, not the endless threat of darkness."
MPs are due to vote around 17.00 this evening.