A source in Serekaniye told Kurdistan 24 that Turkish bombardment killed both a male and a female and injured several others, including a child.
Other sources in Qamishli said an elderly Christian man was killed in an airstrike in the neighborhood of Bashiriye and many others were wounded and transferred to nearby hospitals in the city.
On the western outskirts of the town of Tal Abyad, a man and his wife were killed as artillery shells struck their house, which was totally destroyed.
About nine other civilians were injured in the airstrikes and shelling of the town, local media reported.
The offensive launched by the Turkish army started on Wednesday evening from the town of Serekaniye, also known as Ras Alain in Arabic, then Tal Abyad, Ain Issa, and Qamishli followed.
Read More: Turkish warplanes start shelling northeast Syria, SDF spokesperson says
After two hours of bombardment, Turkish warplanes then targeted the town of Derik, also known as Malikiya, in the far northeastern corner of Syria.
Clashes then erupted between Turkish soldiers fighters of the US-backed, Kurdish-led Syrian Democratic Forces (SDF) in the town of Derbesiye.
In the city of Manbij, west of Euphrates River, Turkish-backed Syrian opposition armed groups controlling the nearby areas of Al-Bab and Jarabulus launched ground attacks on Kurdish-led forces known as the Manbij Military Council (MMC), an SDF ally.
MMC spokesperson Shervan Darwish said they killed four Turkish-backed Syrian rebels in the clashes.
Local media and witnesses report that airstrikes and shelling are still ongoing late into the night in towns and cities along the border with Turkey and that casualties are increasing amid the heavy bombardment.
A White House statement made late on Sunday explained that American forces would withdraw from areas in northern Syria where Turkey has said it would launch a long-threatened military operation targeting US-backed Kurdish forces.
The move drew stern bipartisan criticism of President Donald Trump.
Read More: Trump waffles amid continued criticism, as Turkish attack looms
Amnesty International warned on Wednesday that civilians are at serious risk from the cross-border attacks.
“It is imperative that all parties to this conflict respect international humanitarian law, including by refraining from carrying out attacks on civilians and civilian objects, as well as indiscriminate and disproportionate attacks,” read a statement by the international human rights watchdog.
Read More: Civilians at risk from Turkish military offensive in Syria: Amnesty International
The United Nations Security Council is set to meet on Thursday in response to the attack, international diplomats announced later in the day.
According to Amelie de Montchalin, the French Minister of European Affairs, Britain, France, and Germany called for the meeting shortly after the launch of the operation on Wednesday evening.
Read More: UN Security Council to hold emergency meeting Thursday after Turkey’s military action: diplomats
Several nations have condemned Turkey's actions, including Germany, which called on Ankara "to end the offensive and pursue its security interests peacefully."
Read More: Germany 'sharply' condemns Turkish attack on northern Syria
Editing by John J.