Turkish President Recep Erdogan issued the warning in response to Trump's decision this week to provide arms to Kurdish Peoples' Protection Units, known by the acronym YPG. The rebel fighters are part of a rebel alliance backed by the U.S. government.
"I hope very much that this mistake will be reversed immediately," Erdogan said.
Turkey's resistance to the idea stems from its view that YPG is a threat. Ankara views the group as an extension of the Kurdistan Workers' Party, or PKK, which is considered a terrorist group by both Turkey and the United States.
"We want to believe that our allies would prefer be side by side with ourselves rather than with the terror groups," Erdogan said.
"We are keenly aware of the security concerns of our coalition partner Turkey," Pentagon spokeswoman Dana White said Tuesday, confirming the decision to arm rebels. "We want to reassure the people and government of Turkey that the U.S. is committed to preventing additional security risks and protecting our NATO ally."
For years, rebel fighters have clashed with Islamic State militants and government forces in Syria as a bloody civil war has continued to engulf the Middle Eastern country.
"[The] message to the Trump administration was that Turkey reserves the right to take military action," a senior Turkish official told The Washington Post.
Turkey's military has already carried out strikes against the Kurdish rebels in northern Syria in recent weeks, but could step up the frequency of future attacks.
Erdogan's warning comes a week before he is scheduled to meet with Trump at the White House, on May 16-17.
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