A Turkish security source said the military operation into Syria had begun with air strikes.
"It is an absolute mystery whether Turkey has the will or the capacity to take charge of these imprisoned IS terrorists", Tulin Daloglu, an expert on Turkish-U.S. relations, told Xinhua. Patrolling with Turkey and global forces to protect the Kurds and Turkey is the way to go.
"I call on Turkey as well as the other actors to act with restraint and to stop operations already, as we are speaking, under way", Juncker said. Turkey, he said, will never tolerate the creation of a "terror corridor or a terror state" at its border under any circumstances, regardless of the cost.
In another tweet, Erdogan added that "Operation Peace Spring" would "neutralize terror threats against Turkey" and help facilitate the "return of Syrian refugees to their homes" by establishing a safe zone.
The SDF, which are led by the Kurdish YPG militia, has long angered the Turkish government and Erdogan. The AP could not verify the report independently.
It was hard to know what was hit in the first hours of the operation.
A spokesman for the US-backed Kurdish-led force in northern Syria said Turkish warplanes had started targeting "civilian areas" in northern Syria.
The US-backed, Kurdish-led Syrian Democratic Forces (SDF) said Turkish forces were attacking near the border. It also ignites new fighting in Syria's 8-year-old war, potentially displacing hundreds of thousands.
Founded in 2003, the PYD and its military wing the YPG are the ideological sister group of the Kurdistan Workers Party (PKK), which has fought a decades-long conflict with Turkey.
A Reuters cameraman in the Turkish town of Akcakale saw several explosions across the border in the Syrian town of Tel Abyad. "There are no American soldiers in the area", President Donald Trump said in a statement Wednesday.
In his statement Wednesday, Trump said Turkey is now "responsible for ensuring all ISIS fighters being held captive remain in prison and that ISIS does not reconstitute in any way, shape, or form".
But he also said in Rome that Turkey should not "further destabilize the region" with its military action in Syria.
Italy condemned the offensive as "unacceptable", saying military action in the past always led to terrorism.
Turkey has said it intends to create a "safe zone" in on the Syrian side of the border where it could send back some of the 3.6 million refugees. The Turkish air strikes killed at least five civilians and three fighters from the SDF and wounded dozens of civilians, the SDF said.
A senior administration official said the United States had leaned heavily on the SDF to reduce its fortifications in those areas, under the idea that Turkey would then not invade.
Turkey's Anadolu news agency reported shelling of Kurdish militant positions in the border town of Tal Abyad.
Critics of Mr. Trump's decision fear a Turkish operation could have destabilizing consequences for the region, while both Democrats and Republicans have warned that a Turkish attack could lead to a large number of fatalities among the Kurds, who are holding thousands of captured IS fighters and their families.
Yet former U.S. officials still expect the Kurdish forces to be worn down by Turkish tanks - or forced to flee - in the open deserts of Northern Syria. There was no immediate word on casualties. An activist collective known as Raqqa is being Silently Slaughtered reported an exchange of fire and an explosion.
IS claimed an overnight suicide attack by two of its fighters in its former Syria stronghold of Raqa.