The troops in Syria have been supporting the Kurdish-led Syrian Democratic Forces fight ISIS, but with the militant group all but defeated, the U.S. president made the surprise announcement of a total USA withdrawal on December 19.
Turkey, however, regards the People's Protection Units (YPG) as terrorists. Erdogan has been massing Turkish troops on the Syrian border for weeks, preparing for an invasion to eradicate Kurdish forces that the USA has vowed to protect.
Trump's Twitter posts on Sunday were the the latest in the slow drip-drip of information being released by his administration after his shock December announcement of a troop withdrawal.
A senior figure in Saudi Arabia's royal family, Prince Turki al-Faisal, told the BBC it would have a "negative impact" that would benefit Iran, Russia and Syria's President Bashar al-Assad.
U.S. secretary of state Mike Pompeo is greeted by Adel al-Jubeir, Saudi Arabia's minister of state for foreign affairs in Riyadh on Sunday.
What did Mr Trump say?"Will attack again from existing nearby base if it reforms", the President tweeted on Sunday.
When Pompeo said earlier this month that the US wants to ensure "the Turks don't slaughter the Kurds", Turkey denounced Pompeo, saying he exhibited "a worrying lack of information".More news: Alexandria Ocasio-Cortez Quotes 'Watchmen's' Alan Moore, Slays Nerd Hearts
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Trump did not detail who would create, enforce or pay for the safe zone, or where it would be located.
President Trump's unexpected announcement last month that troops would leave the country has fueled fears that a power vacuum could strengthen ISIS and cede influence in the region to Iran and Russian Federation.
Nor will it stop the United States from trying to get "every Iranian boot out of Syria", Pompeo said.
"If the (pullout) is put off with ridiculous excuses like Turks are massacring Kurds, which do not reflect the reality, we will implement this decision", Turkish Foreign Minister Mevlut Cavusoglu said on Thursday, adding that the fight against the YPG does not depend on "anybody's permission".
Pompeo stressed those fighting alongside the US are not terrorists and deserve protection.
But, he added, "we also know that those fighting alongside of us for all this time deserve to be protected as well". Syria's deputy foreign minister said on Wednesday he was optimistic about renewed dialogue with the Kurds.
"Out of concern for operational security, we will not discuss specific timelines, locations or troop movements", Ryan said.
About 2,000 USA military personnel are reported to be deployed in northern Syria.