The Turkish forces were firing from Turkey's southern province Hatay and the observation point in Syria's Idlib province.
Erdogan has repeatedly accused the YPG of attempting to link Afrin with a large Kurdish-controlled area to the east. Turkey launched its Euphrates Shield offensive in northern Syria in 2016 to push back Islamic State from its border and block the convergence of the Kurdish-run regions.
Erdogan said the new operation would be an extension of Turkey's 2016 incursion into northern Syria, which was aimed at combating the Islamic State group and stemming the advance of US -backed Kurdish forces. The eight-month combat effort officially ended in March 2017.
"In coming days, we will continue our operations in Afrin to clear our southern border" of terrorists, Erdogan told a meeting of his party in Tokat province on Sunday.
Turkish soldiers are now based in rebel-held territory on both sides of Afrin.
The United States views the YPG as the most effective fighting force against ISIL.
"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", Pahon said. YPG played in a prominent role in defeating the group in its former de facto capital in northern Syria late previous year.
Without mentioning the United States, Erdogan said on Sunday he expected allies not to make "mistakes" by "taking sides" with the YPG during the battle for Afrin.
Turkey will interfere "if the terrorists in Afrin do not surrender", Erdogan also said, adding that Turkish forces "are destroying the western wing of this corridor with the Idlib operation".
A senior Syrian Kurdish official said on Sunday that fighting between the YPG and Turkish forces was already under way.
She called Turkey's operation against Afrin a "violation" that "undermines worldwide efforts to reach a political solution in Syria".
He declined to comment on possible "rules of engagement" between the border units and Turkish or Syrian troops. Rojhat Roj said the shelling of areas in Afrin district, in Aleppo province, killed one YPG fighter and injured a couple of civilians on Sunday.
Roj said the Kurdish militia will fight to "defend our gains, our territories". "They will see what we'll do in about a week", Erdogan said. Turkey considers the YPG to be a terrorist group linked to the outlawed Kurdistan Workers' Party (PKK) that has waged a bloody insurgency within its borders.
Turkey and its Western allies, including the US, consider the PKK a terrorist organization.
Since the PKK launched its terror campaign in 1984, an estimated 40,000 people have been killed in Turkey in related violence.
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