'An ally must be reliable': Macron slams Trump's decision to withdraw from Syria
"I deeply regret the decision," Macron said, speaking in the Chad capital N'Djamena.
It comes after Trump ordered a complete withdrawal of US forces from Syria, claiming that the Islamic State group had been defeated.
He also announced that the US military presence in Afghanistan would be "halved", raising fears about the security of the country gripped by a Taliban fight-back.
IS have been reduced to a small strip of land in the east of Syria, but organised a "huge" counter-attack on the US-backed, anti-IS, Syrian Democratic Forces (SDF) this week, once news emerged of the American withdrawal.
The Kurdish-dominated SDF also fear an offensive by Turkish and Syrian regime forces with reports of a build up Ankara's forces on the border.
"To be an ally is to fight shoulder to shoulder," Macron said, adding that France was doing just that in Chad against al-Qaeda and other extremist groups.
"An ally must be reliable, to coordinate with its other allies," he said.
Macron also paid tribute to Defence Secretary James Mattis who resigned this week over Trump's announcement of a Syrian pull-out.
"I want here to pay tribute to General Mattis... for a year we have seen how he was a reliable partner," Macron said at a news conference with his Chadian counterpart Idriss Deby.
In his resignation letter Mattis stated that Trump's world view was irreconcilable with his own.
"My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades' immersion in these issues," Mattis wrote to Trump, who has sought closer ties with Russia and heaped contempt on NATO and other alliances.
"Because you have the right to have a Secretary of Defence whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position."
Another leading US official to resign over the decision was Brett McGurk, the US special envoy to the anti-IS coalition.
He said he would bring forward his departure from February due to the change in Syria policy.
IS, who once controlled large areas of Iraq and Syria, are now confined to a small amount of territory around Hajin.
The Kurdish People's Protection Units (YPG) who form a large part of the SDF have threatened to quit the anti-IS battle if attacked by Turkey.