The onslaught has stirred diplomatic tensions between North Atlantic Treaty Organisation allies Turkey and the United States.
The YPG makes up the majority of fighters in the SDF and is linked to the designated terror organisation the Kurdistan Workers' Party (PKK).
"You will note that there has been no engagement of Turkish aircraft by Syrian regime air defenses", the official went on.
About the Kurdish forces in Afrin, the administration has had little to say except that they have "never been advised by the United States, equipped by the United States, trained by the United States, and have never had any US forces embedded with them", said the senior Trump administration official.
Erdogan said the operation was a "clear warning" to those who did not want to understand Turkey's determination in the fight against terrorism.
More alarming to the USA military is the concern that Erdogan has made clear he has no interest in keeping his military in Afrin.
Erdogan's remarks came on the seventh day of the Turkish incursion into Afrin, which started last Saturday.
Media reported that a group of worshippers left the Friday prayer at Feqıyê Teyran mosque in Diyarbakir's Kayapinar district when the imam praised the Turkish offensive in Afrin and asked the congregation to pray for Turkey's troops involved in the operation. In the intense clashes at the village, nearly three kilometers away from Turkish border, five FSA soldiers were injured while 30 PYD/PKK individuals were neutralized.
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Speaking at a joint news conference with Spain's Prime Minister Mariano Rajoy on Thursday, Stoltenberg explained that Turkey has the right to defend itself against terrorists. The group said in a statement Friday that among the dead are 59 civilians and 43 fighters, including eight female fighters. In addition, 134 civilians had been wounded and at least 66 killed, it said.
In Ankara, a governmental official said anyone supporting the YPG militia will become "a target".
THE FACTS: While the US has provided training, weapons and logistical support for the YPG and the Syrian Democratic Forces it dominates, that aid is not known to have extended west of the Euphrates to Afrin.
During a press briefing, Nauert said that Turkey was dealing with the "PKK instead of fighting Daesh".
Following the Erdogan-Trump telephone talks, the U.S. envoy to the coalition against IS, Brett McGurk, said on Twitter the "prolonged operation risks giving life to ISIS (IS) as it's on verge of defeat".
"How can a strategic partner do this to its partner?"
Syria's main opposition negotiating body said Saturday it will boycott a peace conference in Russian Federation next week, saying it would not lead to a genuine peace track that could end the country's seven-year war.