Speaking in Riyadh, Secretary of State Mike Pompeo said he did not think the threat would change plans to withdraw troops from Syria.
"We will continue to fight against them all". Erdogan called his comments "a serious mistake".
The Kurdish YPG has been a USA ally in the fight against the Islamic State group and it controls swaths of northern Syria.More news: Manchester United beating Tottenham could be good news for Spurs
"We are trying to create an orderly, successful, deliberate departure of our forces". The U.S. provided military training and supplied truckloads of weapons to the YPG, disregarding warnings from Ankara that the YPG is organically linked to the PKK, and partnering with one terrorist group to fight another was not acceptable.
The US-led coalition on Saturday fired more than 20 missiles against militant positions, the Observatory said.
Mr Trump, who has been accused of abandoning the YPG, the Kurdish militia fighting Islamic State with western backing, said he wanted the Kurds to have a 20km "safe zone" with U.S. diplomatic cover.
Mr. Trump's tweet drew a sharp response from Ankara and the Turkish lira lost some 0.84 percent of its value against the dollar on Monday following the USA president's threat. Turkey fights against terrorists, nor Kurds. Cavusoglu said Turkey would not be against such a proposal.
"Russia, Iran and Syria have been the biggest beneficiaries of the long term U.S. policy of destroying ISIS in Syria - natural enemies". For after years of working at odds with USA policy goals in Syria, Erdogan wrote this treatise not to support US objectives, but to capitalize on an unprecedented chance to shape northern Syria and the lives of the Kurds who live there to his liking. And without clear signals from Washington, the Self-Administration of Northeast Syria-an unrecognized statelet carved out of former ISIS territory-is running out of time to negotiate for its future.More news: Canberra to decide Saudi's fate after United Nations refugee ruling
"The withdrawal is based on operational conditions on the ground, including conversation with our allies and partners, and is not be subject to an arbitrary timeline", he said.
Trump also did not say exactly where such a buffer area would be set up.
Turkey, however, regards the People's Protection Units (YPG) as terrorists.
Foreign Minister Jean-Yves Le Drian of France, which is part of the US-led coalition, welcomed what he believed was a slower withdrawal by the United States after pressure from its allies.
For Americans in favor of leaving Syria but with reservations about doing so, do not take solace in Turkish president Recep Tayyip Erdogan's recent op-ed in the New York Times , for it is insincere political theater masquerading as an olive branch.More news: France to crack down on unauthorised protests, Europe News & Top Stories
Abdulaziz Sager, Chairman and Founder of the Gulf Research Center, told Al Arabiya English that Pompeo's statements sent a strong message in assuring that the "Kurds will be protected".