Air strikes and artillery fire erupted at the Turkey-Syria border on Wednesday.
Hours later Turkish troops and their Syrian rebel allies then began a cross-border ground operation that could transform Syria's eight-year-old war.
The British-based Syrian Observatory for Human Rights said 16 members of the Kurdish-led Syrian Democratic Forces militia were killed in the early hours of the Turkish offensive.The invasion has forced Syrian civilians to flee their homes.
"We saw two tanks in front of our village so we left, fearing that our children might get hurt following the shelling of the village. Most residents of the village left, but some young men stayed."
Turkey's President Recep Tayyip Erdogan is calling the attack "Operation Peace Springs".
He said the aim is to create a so-called "safe zone" cleared of Kurdish fighters who they regard as terrorists.
"Together with all our commanders, we will bring down the terror corridor. We are determined. Our combat is not against the civilians. No steps will be taken against the civilians."
The assault began three days after US President Donald Trump pulled American troops - who had been effectively serving as a buffer - out of the area.
Today (Thursday) Mr Trump called the Turkish assault a "bad idea" and threatened to devastate Turkey's economy if Ankara's incursion in Syria eliminated the Kurdish population there.
"I will wipe out his economy if that happens. I've already done it once with Pastor Brunson. I'm sure that the, I hope that he will act rationally."
World powers fear the Turkish action could open a new chapter in Syria's war and worsen regional turmoil.
United Nations Spokesman Farhan Haq said UN Secretary General Antonio Guterres is deeply concerned by the escalating situation in Syria.
“The Secretary-General is very concerned by the recent developments in northeast Syria. 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 United Nations Security Council are holding an emergency meeting to discuss Turkey's military offensive.
