Macron says 'an ally must be reliable' after US Syria pullout announcement

French President Emmanuel Macron on Sunday criticised US President Donald Trump's decision to withdraw American forces from Syria, saying "an ally must be reliable".

Speaking in the Chad capital N'Djamena, Macron said "I deeply regret the decision" by Trump to pullout US troops.

Trump last week ordered a complete troop pullout from Syria, asserting that the Islamic State group had been defeated, and a significant withdrawal from Afghanistan.

"To be an ally is to fight shoulder to shoulder," Macron said, adding that France was doing just that in Chad in the fight against jihadist groups.

"An ally must be reliable, to coordinate with its other allies," he said.

Macron also paid tribute to Defence Secretary James Mattis, who said he was resigning on Thursday after Trump's Syria announcement.

"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.

Mattis, 68, was one of Trump's first cabinet picks and has spent nearly two years at the Pentagon.

In his resignation letter, Mattis spelled out to the world what seemed obvious to many observers: 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 Defense 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."