The Holy Father said: "In these cases, where there is an unjust aggression, I can only say it is legitimate to stop the unjust aggressor.
"I emphasise the verb 'stop'; I'm not saying 'bomb' or 'make war,' just 'stop'. And the means that can be used to stop them must be evaluated."
The Pontiff was speaking during a press conference on his flight back from his trip to South Korea.
The Vatican has already indicated it was in favour of military action to stop IS fighters (formerly ISIS) from advancing across Iraq any further.
Militants have taken over large parts of the country forcing thousands of Christians to flee their homes.
Fighters gave believers in one town an ultimatum to convert to Islam, pay a tax or be killed.
Pope Francis added: "One nation alone cannot judge how to stop an unjust aggressor. After the Second World War there was the idea of the United Nations. It is there that this should be discussed. Is there an unjust aggressor? It would seem there is. How do we stop him? Only that, nothing more.
"They talk to me about the Christians, the poor Christians. It's true, they suffer. The martyrs, there are many martyrs. But here there are men and women, religious minorities, not all of them Christian, and they are all equal before God.
"To stop the unjust aggressor is a right that humanity has, but it is also a right that the aggressor has to be stopped so that he does not do evil."
The Pontiff also indicated he was considering a visit to Iraq. "Yes, I am willing," he said when asked by reporters.
He said: "But I think I can say this, when we heard with my collaborators about the killings of the religious minorities, the problem at that moment in Kurdistan was that they could not receive so many people.
"It's a problem that one can understand. What can be done? We thought about many things. First of all a communique was issued by Fr. Lombardi in my name.
"Afterwards that communique was sent to all the nunciatures so that it be communicated to governments.
"Then we wrote a letter to the Secretary General of the United Nations.
"Many things .... And at the end we decided to send our personal envoy—Cardinal Filoni, and I said if it were necessary when we return from Korea we can go there.
"It was one of the possibilities. This is my answer. I am willing [to go there]. At the moment it is not the best thing to do, but I am ready for this."