Fighters have been encircling Raqqa, with help from USA forces advising them, and American officials are weighing more assistance to help them drive IS from the city. The US views the two Kurdish groups as distinct organizations.
The Obama administration had approved military support for the YPG and other Kurdish militias in early 2015, as they fought to drive the Islamic State from the Syrian town of Kobane, which straddles the Turkish border.
The Kurdish elements of the SDF are from the Kurdish Peoples' Protection Units (YPG) and they have been the main faction fighting ISIL, which stands for Islamic State of Iraq and the Levant, and is also known as ISIS, on the ground in Syria.
The US official said several administration brass, including Defense Secretary James Mattis, talked to Turkish interlocutors Tuesday to explain the US decision.
The YPG forms the backbone of the SDF's forces.
The Pentagon sought to assure Turkey that the decision to arm the YPG would not translate into USA support for a Kurdish enclave in Syria. Erdogan has said that he will now send his troops, and Syrian rebels fighting with them, to Raqqa, eliminating the need for the United States to depend on Kurdish forces.
"We're keenly aware of the security concerns of our coalition partners in Turkey", said White House press secretary Sean Spicer on Tuesday.
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"We want to reassure the people and government of Turkey that the U.S.is committed to preventing additional security risks and protecting our North Atlantic Treaty Organisation ally", according to the statement.
In Denmark, Mattis was asked about the way forward with Turkey over the Raqqa issue.
"Our intent is to work with the Turks alongside one another to take Raqqa down, " Mattis said during a news conference in Denmark. "That's not to say we all walk into the room with same appreciation of the problem".
For the US administration, which is skeptical that the Turkish-backed force is large enough or sufficiently trained, the decision sets Trump's wish for quick battlefield victories against the need to maintain its strategic alliance with Turkey. The SDF is expected to spearhead the coming assault on the Islamic State capital of Raqqa. Adrian Rankine-Galloway replied, "We encourage all forces to remain focused on the Defeat-ISIS fight and concentrate their efforts on ISIS".
There are now more than 1,000 American service members in Syria, after the Pentagon sent about 400 additional Marines to Syria in March to prepare for the Raqqa offensive.
It was not immediately clear whether the decision by Trump means the YPG will receive heavier weapons, including anti-tank missiles and armored vehicles. However, over the last several weeks, Ankara has conducted a unilateral air campaign against Kurdish targets in Syria.