Emmanuel Macron bemoaned Donald Trump's decision to withdraw US troops from Syria.
The French president said: "An ally should be dependable."
During a press conference in Chad he said: "I very deeply regret the decision made on Syria."
"To be allies is to fight shoulder to shoulder. It's the most important thing for a head of state and head of the military."
As well as this, President Macron stressed the importance of the work of the Kurdish-led Syrian Democratic Forces, which has captured large parts of northern and eastern Syria from Islamic State.
"I call on everyone ... not to forget what we owe them," he said.
President Trump has faced criticism from numerous angles over the move, with high profile resignations in the US government over the situation.
However, he has been steadfast in defending the choice, writing on Twitter that anyone else who did the same would be "the most popular hero in America".
On Saturday a US State Department official confirmed Brett McGurk, special envoy for the global coalition to defeat ISIS, handed in his resignation.
The 45-year-old joined Defence Secretary Jim Mattis in an exodus of experienced national security officials over the withdrawal.
Earlier this week, Mr Trump released a video message on Twitter to announce the surprise decision.
“We have won against ISIS [Islamic State],” said President Trump.
“We’ve beaten them and we’ve beaten them badly. We’ve taken back the land and now it’s time for our troops to come back home. I get very saddened when I have to write letters or call parents or wives or husbands of soldiers who have been killed fighting for our country. So our boys, our young women, our men, they’re all coming back and they’re coming back now. We won, and that’s the way we want it.”
President Trump is acting to pull all 2,000 US troops from Syria while he has now declared victory over the Islamic State, contradicting his own experts' assessments.
