The US delivers its first weapons shipment to the YPG in a move likely to anger Washington's NATO ally Turkey, which sees the move as a threat to its national security.
The United States has started distributing arms to the YPG in northern Syria, Pentagon spokesman Major Adrian Rankine-Galloway said in a statement on Tuesday.
Washington announced earlier this month that it would start directly arming "Kurdish elements" within the US-backed Syrian Democratic Forces (SDF) based on authority given by President Donald Trump.
The first official shipment to the YPG included small arms and vehicles from the US military, the Pentagon spokesman said.
There was no immediate reaction from Turkey, which previously warned the US that its decision to arm the YPG could end up hurting Washington.
Turkey considers the YPG to be the Syrian branch of the PKK, which has waged an insurgency in its borders since 1984 that has claimed some 40,000 lives.
Although both Turkey and the US deem the PKK to be a terrorist organisation, the US views the YPG in Syria to be a key ally in the battle against Daesh.
The US is preparing to launch an operation to liberate the Syrian city of Raqqa from Daesh with the SDF, an alliance of various factions in northern Syria among which the YPG is the main component.
But Washington's NATO ally Turkey worries that advances by the YPG in northern Syria could inflame the PKK insurgency on Turkish soil.
Turkey has also voiced concern that weapons given to the YPG would end up in the hands of the PKK.
US officials have told Reuters that the United States was also looking to boost intelligence cooperation with Turkey to support its fight against the PKK.
It was unclear if the effort would be enough to soothe Turkey, however.
TRT World's Ediz Tiyansan has more.