Trump is expected to officially inform Erdogan of his decision Tuesday, when he visits the White House.
In a major policy shift, the Pentagon announced Tuesday it would directly arm Syrian Kurdish fighters for the first time to help expedite the capture of the so-called ISIS capital of Raqqa - against the wishes of North Atlantic Treaty Organisation ally Turkey, which considers the group terrorists.
The SDF, including its Kurdish elements, "are the only force on the ground that can successfully seize Raqqa in the near future", she said in a statement announcing the plan.
Turkey has continued to lobby the Trump administration to change course in the days leading up to President Recep Tayyip Erdogan's U.S. visit, dispatching top Turkish officials, including General Hulusi Akar, the military chief of staff, and Hakan Fidan, the intelligence chief, to Washington.
Turkey views those Kurdish fighters in Syria as terrorists, or as allied with terrorists, due to their connection to the Kurdish PKK militant group, which wants to carve out its own independent state within Turkey.
The US sees the 50,000-strong Syrian Democratic Forces as the most effective force fighting ISIS in Syria and has armed the non-Kurdish Arab elements of that group for some time.
Tillerson said. "Once the ISIS threat has been reduced or eliminated, I think we can turn our attention directly to stabilizing the situation in Syria".
"We are keenly aware of the security concerns of our coalition partner Turkey", said White in the statement. Kurdish forces have always been considered among the most reliable groups in the fight against ISIS.
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But a group of Kurdish fighters, commonly referred to as the People's Protection Units or YPG, is considered a terrorist organization by Turkey, another US ally fighting ISIS.
The Turkish president, fresh from winning the controversial April 16 referendum on enhancing his powers, has indicated that the rewards for Washington in breaking up with the YPG could be high by spurring Turkish involvement in a joint operation to take the IS fiefdom of Raqa. The official has no confirmed or disclosed just how soon arms would start being delievered to Kurdish fighters.
Once the order comes to the Pentagon, the USA can begin providing the Syrian Kurds with arms and equipment fairly quickly since some equipment is pre-positioned.
The Pentagon said that it would continue prioritizing the arming of Arab forces in Syria, but acknowledged Ankara's concerns in the statement announcing the decision.
"That's not to say we all walk into the room with exactly the same appreciation of the problem or the path forward", Mattis told reporters after meeting with officials from more than a dozen nations also fighting ISIL.
Tensions escalated last month when Turkey launched air raids on Kurdish bases in Syria and Iraq.
"We're going to sort it out", he replied.