Turkey is massing troops near a town in northern Syria held by a U.S.-backed and Kurdish-led force, a war monitor and Turkish media said Sunday.

The buildup comes even though Turkey said it would delay a promised offensive in eastern Syria in the wake of President Donald Trump's decision to withdraw U.S. troops earlier this week.

The Trump decision surprised his allies and own experts, sparking the resignation of two of his top aides.

On Sunday, French President Emmanuel Macron said he "deeply regrets" Trump's decision to pull U.S. troops out of Syria and warned it could have dangerous consequences.

Macron showered praise on U.S. Defense Secretary Jim Mattis, who quit in the wake of Trump's unexpected move. "An ally should be reliable, coordinate with other allies. Mattis understood this," Macron said during a trip to Chad.

Macron said that the troop withdrawal endangers Kurdish fighters, who were instrumental in the U.S-led coalition's fight Islamic State militants.

"We should not forget ... what we owe to those who died on the ground fighting terrorism," he said, referring to the Kurdish-led Syrian Democratic Forces. "The SDF is fighting against the terrorism that fomented attacks against Paris and elsewhere ... I call on everyone not to forget what they have done."

Macron did not say what France's military will do next in Syria. Kurdish officials met with a French presidential adviser Friday, and one asked France to play a larger role in Syria following the American withdrawal.

Turkey however welcomed the Trump's decision. Ankara views the Kurdish militia at the heart of SDF as an extension of the insurgency within its borders. Turkey's President Recep Tayyip Erdogan has vowed to dislodge the Kurdish fighters from along its border with Syria.

The U.S. had partnered with the Syrian Kurdish militia to drive out IS group since 2014. But the partnership had soured relations between Ankara and Washington.

Allaying some of Turkey's fears was a deal reached in June according to which U.S. and Turkish troops would patrol the area around the Syrian town of Manbij. But Ankara says the U.S. and the Kurdish militia didn't live up to their end of the deal and that it would start an offensive in eastern Syria to drive out the militia. Turkey already has troops in northwestern Syria and has backed Syrian fighters there to clear towns and villages of IS militants and Kurdish fighters.

Erdogan said he would delay the eastern Syria offensive and would work on plans to clear out IS from the region.

On Sunday, the Turkish IHA news agency reported that a convoy of Turkish troops — a commando unit — had been sent into Syria overnight.

The Syrian Observatory for Human Rights said the reinforcements were sent to the front line with Manbij, where U.S. troops have been based. The Observatory said 50 vehicles crossed into Syria — carrying troops and equipment.

The spokesman for the Kurdish-led Manbij Military Council, Sharfan Darwish, said Turkish reinforcements have arrived in the area. "We are taking necessary measures to defend ourselves if we are attacked," he said without elaborating.

On Sunday, U.S. troops based around Manbij patrolled the town and surrounding area and were photographed speaking with the residents.

A spokesman for the Turkey-backed Syrian opposition fighting group said the continued Turkish and allied forces buildup is to prevent Syrian government troops from taking advantage of the tension in the area to seize territory.

Youssef Hammoud, spokesman for the Syrian opposition fighters, accused the Kurdish militia of reaching out to the Syrian government to replace U.S. troops if they withdraw.

Darwish dismissed the claims as "untrue," calling them "old accusations" from the rival Syrian groups.

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Associated Press writers Zeynep Bilginsoy in Istanbul and Angela Charlton in Paris contributed to this report.