President Donald Trump meanwhile denied the Kurds had been abandoned by the USA pullback, praising Turkey as a trade partner and North Atlantic Treaty Organisation ally in a softening of his tone hours after threatening Ankara's economy if it acted "off limits" in Syria.
"We are helping the Kurds financially/weapons!", he added.
Ankara regards the YPG as a terrorist group affiliated with the Kurdistan Workers' Party (PKK), which has fought the Turkish state for more than three decades. "The TSK will never tolerate the establishment of a terror corridor on our borders. All preparations for the operation have been completed", the Turkish Defence Ministry said on Twitter yesterday. The below is from Akcakale, on the Turkish border with Syria.
The U.K. -based, pro-opposition Syrian Observatory for Human Rights also reported on Tuesday's shelling at Ras al-Ain, along with an artillery attack on the town of Marimeen and the vicinity of the Menegh Military Airport in Syrian Democratic Forces-held territory in northern Aleppo province.
A Turkish invasion looks more likely after President Donald Trump's sudden announcement that USA troops, who had fought alongside the Kurds against Islamic State group, would withdraw from the area.
The comment appeared to be a response to Mr Trump's earlier threat to target the Turkish economy, reportedly an attempt by the president to placate domestic criticism, including from Republican Senate Majority Leader Mitch McConnell.
Trump on Monday said that he has "told Turkey that if they do anything outside of what we would think is humane... they could suffer the wrath of an extremely decimated economy".
"People are scared. When we used to see USA troops in the streets of Tel Abyad, we would feel safe; they were here to protect us".
Turkey is preparing for an offensive into northern Syria after U.S. President Donald Trump announced Sunday he was pulling back United States troops who had served as a buffer preventing the long-planned attack on Kurdish forces.
In a tweet, Fahrettin Altun said that Kurdish militants there could either defect or Ankara would have to "stop them from disrupting" Turkey's struggle against the Islamic State militants.
The captives are held in SDF facilities south of a safe zone initially proposed by Turkey.
Al Jazeera's Charles Stratford, reporting from Akcakale along Turkey's border with Syria, said: "All evidence suggests that the operation could be imminent".
The uproar prompted a White House phone briefing with reporters late Monday afternoon, in which a senior administration official insisted the president's decision was neither abandonment of the Kurds nor the US withdrawal of its last troops from Syria - something the president himself alluded to in tweets Monday.
AP journalists on the Turkish side of the border overlooking Tal Abyad saw Turkish forces crossing into Syria in military vehicles Wednesday, though there was no official statement from either side that the offensive had begun.
Britain yesterday said it was "deeply concerned" by Turkey's plans and would not support the move.
The SDF said Turkish forces were shelling one of their positions in the border town of Ras al-Ain - one of the places from which US troops withdrew earlier this week, according to a British-based group monitoring the war.
But the Kurds say Ankara's real goal is to dilute their demographic dominance of the northeast with an influx of mostly Sunni Arab refugees from other parts of the country now living in southwestern Turkey.
Dozens of military vehicles were sent to the area following a United States troop withdrawal from north-eastern Syria.