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

In an abrupt policy shift, Trump announced on Wednesday that Washington would withdraw the roughly 2,000 U.S. troops in Syria, upending a pillar of American policy in the Middle East and alarming U.S. allies.

"I very deeply regret the decision made on Syria," Macron said during a news conference in Chad.

"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. "An ally should be dependable."

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.

U.S. officials justified the decision by saying Islamic State had been defeated.