Laurent Fabius called for international mobilisation against “this transnational danger that could reach all the way to our soil”.
France says it will join a US-led coalition and is sending arms to Kurdish authorities to fight the militants.
The French president and foreign minister are going to Iraq tomorrow and hosting an international conference on Monday on how to stop the group and help Iraq.
Mr Fabius said in a speech in Paris that “in Iraq… we will take part in military air action if necessary. The situation in Syria is different.
“Some in the media say France is ready to act in Iraq, but not Syria. No. We must act in both cases, but not with the same modalities.”
Mr Fabius said Syrian president Bashar al-Assad could not be a partner in the fight against Islamic State as he was complicit with the militants.
“That is why we will continue to help the moderate Syrian opposition that is fighting Islamic State and Assad at the same time,” he said.
President Barack Obama was last night due to give a major speech to outline a greater military and political effort to combat militants in Syria and Iraq.
He was to urge Congress to quickly give him authority to arm moderate Syrian opposition forces fighting Mr Assad.
Officials said Mr Obama will press forward with other elements of his plan without formal authorisation from politicians.
That could include air strikes in Iraq and possibly Syria.
Other elements of Obama’s plan, which he was to lay out in a televised speech, included increased support for Iraqi security forces, as well as military and diplomatic commitments from partners in Europe, the Middle East and elsewhere.
The speech is rare because presidents seldom ask for television coverage at peak-time viewing, doing so only to address the most critical issues.
For Mr Obama, a sustained US intervention in the Middle East is at odds with the vision he had for the region when he ran for president on a pledge to end the war in Iraq, where the role of American forces drew to a close nearly three years ago. The timing of his announcement was all the more striking, just hours before memorial events on 9/11.
Among the president’s most urgent priorities will be seeking backing from Congress to arm more moderate elements of the opposition fighting Mr Assad. The president asked earlier this year for a $500 million (£309m) train-and-equip programme, but the plan stalled in Congress.
The US has been running a smaller CIA programme to train the rebels, but Mr Obama wants a more overt military effort that could involve training locations in countries near Syria.
With Mr Obama ruling out sending in US troops, bolstering Iraqi forces and the Syrian opposition will be crucial to root out Islamic State (IS). Air strikes could also help those forces make gains.
US secretary of state John Kerry travelled to the region yesterday for discussions in Saudi Arabia and Jordan. The US is already striking against IS targets inside Iraq, at the invitation of the Iraqi government and without formal authorisation from Congress.
But the scope of the mission has been relatively limited to air strikes that help protect American interests and prevent humanitarian crises.
US officials said Mr Obama was expected to loosen those limitations and open a broader counterterrorism campaign against the militants.