The artillery strikes, which also targeted YPG gun and sniper positions, were aimed at sites far from residential areas, the source said.
The Britain-based Syrian Observatory for Human rights reported that people had begun fleeing the border town of Tal Abyad.
Washington Post columnist David Ignatius reported on Tuesday night that Turkish forces would strike Tel Abyad and Ras al-Ayn, two of the zones included in the joint "security mechanism" that Trump made a decision to abandon on a call with Erdogan on Sunday.
The air and artillery strikes have been reportedly targeting the vicinity of the Syrian border towns of Tel Abyad and Ras al-Ayn.
Soldiers with heavy equipment have removed concrete sections of the border and howitzers have been positioned behind earth embankments on the Turkish side of the border near the town of Akcakale. There were no immediate reports of casualties there.
World powers fear the action could open a new chapter in Syria's war and worsen regional turmoil.
President Tayyip Erdogan has recently talked about pushing even deeper into Syria, beyond the proposed "safe zone" region to the cities of Raqqa and Deir al-Zor, in order to allow still more refugees to return to Syria.
In the build-up to the expected offensive, Syria had said it was determined to confront any Turkish aggression by all legitimate means.
In an op-ed in The Washington Post on Wednesday, Fahrettin Altun, Turkey's communications director, wrote that Turkish forces, with their Syrian rebel allies, "will cross the Turkish-Syrian border shortly".
"Turkey has no ambition in northeastern Syria except to neutralise a long-standing threat against Turkish citizens and to liberate the local population from the yoke of armed thugs".
Meanwhile, in parts of the U.S. government, anger continued to fester against Turkey for the planned attack after the extensive preparations made to set up a security mechanism to placate Ankara's suspicion of Kurdish units.
Turkey has been massing troops for days along its border with Syria and vowed it would go ahead with the military operation and not bow to the U.S. threat.
According to multiple current and former USA officials, the White House's announcement blindsided not just America's Kurdish partners but nearly everyone - senior officials at the Pentagon, the State Department and the White House, lawmakers on Capitol Hill, and US allies in Europe and the Middle East.
Meanwhile, UK and France are planning to call for an emergency meeting of the UN Security Council to discuss the situation, and European Commission President Jean-Claude Juncker has urged Turkey to stop the airstrikes. "The Department's position has been and remains that establishing a safe zone in northern Syria is the best path forward to maintaining stability".
"The difficulty we are going through is this, that we have partnered in dignity and clarity and friendship with American forces for 5 years and it is really hard for us to accept an end to this partnership in an unworthy way", SDF spokesman Mustafa Bali told VOA's Kurdish service.
While a Kurdish-led operation earlier this year saw the death of IS's territorial caliphate, the organisation is not dead and sleeper cells have been active in several parts of Syria and Iraq.
The statement said the mobilization would last for three days.
Turkey was taking over leadership of the fight against ISIS in Syria, he said.
Fahrettin Altun, the Turkish presidency's communications director, called on the worldwide community in a Washington Post op-ed published Wednesday "to rally" behind Ankara, which he said would also take over the fight against the Islamic State group.
At least one member of the Kurdish-led force known as the Syrian Democratic Forces was killed in the Turkish bombardment, Kurdish activists and a Syria war monitor said.