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 U.S., however, views the Kurdish forces in Syria as some of the country's best fighters, and necessary to the upcoming offensive to retake Raqqa, the northeast Syrian city which ISIS considers its capital.
US defense officials have been mulling the move for months but waited to announce it until after a referendum that enabled President Recep Tayyip Erdogan to consolidate his power in April. They said the US would not provide artillery or surface-to-air missiles. US-backed fighters have captured 80 percent of Syria's Tabqa from the Islamic State group, a monitor said on May 1, a week after they first entered the town.
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.
A Pentagon source told the BBC the equipment would include ammunition, small arms, machine guns, heavy machine guns, construction equipment such as bulldozers and armoured vehicles.
"The Turks are going to be unhappy with this because they, in no way, want to see any kind of bolstering of strength of Kurdish forces in that area", Al Jazeera's Charles Stratford said, reporting from Gaziantep along the Turkey-Syria border.
The SDF, which comprises Kurdish and Arab militias, is supported by elite U.S. forces and air strikes from a US-led coalition.
Mattis acknowledged the war in Syria had not left the United States with easy choices but he voiced confidence that Washington and Ankara would get past the impasse.
Pentagon chief Jim Mattis, who arrived in Vilnius late today as part of a Europe trip, earlier attended a summit in Copenhagen for senior leaders from the top 15 countries in the anti-IS coalition, including Turkey.
While there is no mention in the statement of arming the Kurds - it only calls to "equip" them - USA defense officials told Fox News the plan includes supplying machine guns, small arms, ammunition, bulldozers and armored vehicles such as the M1117 Guardian.
"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.
Mattis, speaking before the announcement, sounded upbeat about working through tensions with Ankara when he spoke to reporters in Denmark after talks with coalition officials, including from Turkey.
The United States has not yet disclosed a decision on whether it will arm YPG fighters to take Raqqa, Islamic State's de facto capital, and Jim Mattis declined to answer that question after talks with USA allies in Denmark.
"We are going to further accelerate this fight", Mattis said. "It's not a good situation", he said.