PARIS — President François Hollande said Thursday that France was willing to join the United States-led campaign against the Islamic State, including airstrikes against militants in Iraq, saying the world was confronting unremitting brutality.

Observing that the militants had conquered territory in Iraq and Syria, Mr. Hollande said the group had been able to grow partly because of international inertia. But he emphasized that France’s intervention would be limited to providing air support, including strikes.

The Islamic State, also known by the acronyms ISIS or ISIL, “massacres anyone who resists it; hunts minorities, notably Christians; commits atrocities against civilians; decapitates journalists; crucifies opponents; kidnaps women,” he said. “That is the movement we are up against.”

Mr. Hollande traveled to Baghdad last week to help mobilize support for military strikes against Islamic militants. On Thursday, he said he had met with his top military advisers and had agreed to Iraq’s request for air support to reinforce Iraqi soldiers and Kurdish fighters. He said French fighter jets would strike once targets had been identified. “That means in a short time frame,” he said.