Turkish officials have expressed frustration with the USA for leading a coalition to set up a new force of 30,000 personnel along the Syrian's northern border, Reuters reported Sunday.
"The slightest disturbance on the border would be the signal for us to take a step", Mr Erdogan said in a televised speech.
Turkish soldiers are now based in rebel-held territory on both sides of Afrin.
The issue is among many causing tense relations between Ankara and Washington, though Turkish officials said in November previous year that president Donald Trump had told them the United States would no longer supply weapons to the YPG.
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. The Kurds received US backing as the most effective local proxies against Islamic State in Syria, and the policy of arming them stoked tension between the North Atlantic Treaty Organisation allies.
"More Kurds will serve in the areas in northern Syria".
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"I hope that during an Afrin operation, these powers will not make the mistake of appearing to be on the same side as a terror organisation", Mr Erdogan said in an apparent reference to the U.S. during a rally in the northern Turkish city of Tokat.
"I hope these forces will not make a mistake of appearing on the same stage with the terrorist organisation during the Afrin operation", the president said. "It's absolutely not possible to accept this", Kalin said, and he repeated that Turkey would defend itself.
Roj, the YPG spokesman, said the Kurdish militia would fight to "defend our gains, our territories".
YPG is considered by Turkey to be a "terrorist group" with ties to the banned Kurdistan Workers' Party (PKK), which has waged a decades-long insurgency inside the country.
She called Turkey's operation against Afrin a "violation" that "undermines worldwide efforts to reach a political solution in Syria".
Since December, Ankara has reinforced its southern border in Hatay and sent armoured vehicles, tanks and howitzers, sources told Hurriyet daily.