On April 30, USA forces accompanied by Kurdish People's Protection Units (YPG) began patrols along the Turkey-Syria border, acting as a buffer after Turkish airstrikes in the area killed 20 Kurdish fighters.
But Turkish Foreign Minister Mevlut Cavusoglu slammed the move, saying "every weapon that reaches their hands is a threat toward Turkey", and that Turkey's president would raise the issue during a visit to Washington planned for next week.
The Pentagon yesterday said President Donald Trump had approved arming Kurdish fighters ahead of an eventual offensive to recapture the Islamic State group's Syrian stronghold of Raqa.
"The fact that the United States is handing over arms to the YPG, which even they have recognised is a terror group, does not change reality, this can not be accepted", deputy PM Nurettin Canikli told Turkish television. But Ankara regards the group as a terror outfit, as it does the Syrian branch of the Kurdistan Workers' Party, who have waged an insurgency since 1984 inside Turkey that has left tens of thousands dead.
Mattis expressed optimism that Washington and Ankara would work through tensions after a half-hour discussion with Turkish Prime Minister Binali Yildirim in London, which he described as "honest, transparent and helpful".More news: God Brought You Back, Aisha Buhari Tells Freed Chibok Girls
"Both the PKK and YPG are terrorist organisations and they are no different apart from their names", he said.
But the U.S. is convinced they are the most effective local fighting force against IS.
There was no immediate reaction from Turkey, whose president, Tayyip Erdogan, is expected to meet Trump in Washington next week.
"The Americans have pushed Ankara into a corner where no military operations against the presence of YPG in Syria are possible and, second, Ankara from now on will have to live with the fact the YPG is a political organization like any other that is fighting Islamic State", said former senior Turkish diplomat Aydin Selcen.More news: More details of updated BMW 4 Series
Last month, Erdogan said if Turkey and the United States joined forces, they could turn Raqa into a "graveyard" for jihadists.
The YPG said the USA commitment to the group, and its umbrella coalition the Syrian Democratic Forces, will allow it to expand its operations against IS.
In a brief readout of the approximately 30-minute meeting, a Pentagon statement said the two men discussed a range of bilateral security issues.
"We want to believe that our allies would prefer be side by side with ourselves rather than with the terror groups", Erdogan said.
Turkish media said the three met McMaster at the White House on Monday but gave no details over the content of the talks.More news: New Orleans removes 2nd of 4 Confederate statues before dawn
The full liberation comes after a battle that started March 22 with the US airdropping local ground forces behind enemy lines - a first in the Syrian conflict.