French President Emmanuel Macron slammed US President Donald Trump over his decision to withdraw from Syria and suggested he was mistreating his allies.

"To be allies is to fight shoulder to shoulder," he said. "An ally should be dependable."

Trump abruptly announced last week that he was pulling all 2,000 US troops from Syria, going against own State Department policy and sparking backlash from both parties and lawmakers.

The withdrawal was reportedly the last straw for Defence Secretary James Mattis, who said in his resignation letter that the US should be "treating allies with respect."

French President Emmanuel Macron slammed President Donald Trump's withdrawal of US troops from Syria and suggested that he was not reliable, saying that "an ally should be dependable."

Trump abruptly announced last week that all 2,000 US troops will be withdrawing from Syria, in a move that was criticised by Democrats, Republicans, lawmakers, and Conservative commentators.

Macron said that he regretted Trump's decision.

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

"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."

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Macron also praised the work the Kurdish-led Syrian Democratic Forces, which freed large parts of northern and eastern Syria from ISIS. "I call on everyone ... not to forget what we owe them," he said.

Trump's decision to withdraw was met with alarm across Europe. German foreign minister Heiko Maas in a statement that the threat of ISIS is "not over" and said that Trump's decision "could hurt the fight against the IS and endanger what has been achieved."

Trump's decision is a move that many of the president's advisers had warned against, and it was reportedly the final straw for Defence Secretary James Mattis, who has announced his resignation last week.

In his resignation letter, Mattis said that the US should be "treating allies with respect."

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Brett McGurk, the top US official leading the fight against ISIS, resigned early over Trump's decision. He told his colleagues in an email obtained by The New York Timesthat he could not in good conscience carry out Trump's orders.

"The recent decision by the president came as a shock and was a complete reversal of policy that was articulated to us," he wrote.