French President Emmanuel Macron said on Sunday he deeply regretted U.S. President Donald Trump's decision to withdraw U.S. troops from Syria.

"An ally should be dependable," Macron said during a press conference in Chad. "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," he said.

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

On Wednesday, Washington announced it will be withdrawing all of its troops from Syria, following a conversation between President Recep Tayyip Erdoğan and U.S. counterpart Trump over an imminent Turkish cross-border operation to eliminate PKK-affiliated People's Protection Units (YPG) from northern Syria.

It has been reported that the U.S. still has about 2,000 troops in Syria, many of whom are working in close cooperation with Syrian Democratic Forces (SDF).