The Kurdish People's Protection Units, or YPG, are a key part of the US -backed Syrian Democratic Forces.
As U.S. forces arrived in the city to visit checkpoints, the Kurdish fighter can be heard in the video saying, "Turkey forces want to enter the area".
Mattis said the USA has made it clear all along that, "we're going to equip them for the fight. The Coalition recognizes the threat the PKK poses to Turkey, but Turkey can not pursue that fight at the expense of our common fight against terrorists that threaten us all", he said, in reference to the war on ISIS jihadists.
The U.S. Secretary of Defense James Mattis told his Turkish counterpart Fikri Işık at the meeting in Brussels that their support for and cooperation with the PKK's Syrian affiliate the People's Protection Units (YPG) militia was out of necessity and only temporary.
The YPG has been armed by the U.S. in order to fight the Islamic State (ISIS) in Syria.
"The Turkish state from the north has been gathering its troops with the Syrian groups who call themselves the Syria opposition for a week", according to Hevi Mustafa, co-chair of the executive council of the self-proclaimed Kurdish Afrin Canton in northern Syria.
It depends, he said, on the battle and what weapons the Kurdish fighters need.
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This may have already happened: Jenan Moussa, a Middle Eastern news correspondent for Dubai-based Al Alan TV, concluded an interview with U.S. Special Presidential Envoy for the Global Coalition to Counter ISIS Brett McGurk by announcing on Twitter that she was told by "reliable sources" that the Turkish army has fired "various times" in the direction of U.S. troops stationed along the border. The US has not publicly revealed the amount or even specific types of weapons provided for the Kurds. The officials said that artillery and surface-to-air missiles were not provided.
Mattis also said that as the fight against IS grows more complex and moves into the Euphrates River Valley, it will require more deconfliction with the Russians.
Turkey has been backing the FSA in the Syrian civil war.
Syrian, Russia, U.S. and coalition aircraft all fly in the increasingly crowded skies over Syria.
"We side with the territorial integrity of Syria", he added.
Asked if the various groups would carve up the river valley into controlled segments, Mattis said it may not look that neat. "So it may look a little more squiggly", Mattis said.