Turkish troops have entered the area "east of the Euphrates," targeting a Kurdish-led military alliance.
Video footage showed civilians fled border towns filled with smoke and jet trails were visible in the sky.
At least 15 people, including 8 civilians, have been killed so far, according to the London-based Syrian Observatory for Human Rights.
Turkish President Recep Tayyip Erdogan says the operation is to create a "safe zone" without Kurdish forces which will also shelter Syrian refugees.
He then claimed that the mission will bring peace to the area by preserving Syria's territorial integrity and liberating local communities from terrorists.
The Kurdish-led Syrian Democratic Forces earlier vowed to resist any Turkish advance.
President Trump on Wednesday called the Turkish offensive "a bad idea" after essentially giving the invasion the green light following his abrupt decision to pull U.S. forces out of that part of Syria.
The international community has also raised concerns over the Turkish aggression.
"Any military operation must fully respect the UN Charter and international humanitarian law. Civilians and civilian infrastructure should be protected in accordance with international law."
The UN Security Council will meet on Thursday to address Turkey's military operation, and European Commission President Jean-Claude Junker has pressed Turkey to halt its military actions.
France, Germany and the UK also released a joint statement,calling Turkey's operation an act of "jeopardizing security and humanitarian efforts."
KIM Mok-yeon, Arirang News.