Around 500 Syrian fighters graduated on Saturday from a U.S.-led training course aimed at establishing a controversial "border security force" in the country's north.

Last week, the U.S.-led coalition battling Daesh (ISIS) announced it had begun forming a 30,000-strong security force to patrol territory captured from Daesh.

About half its fighters would hail from the Syrian Democratic Forces (SDF), the alliance of Kurdish and Arab fighters that has emerged as Washington's best ally against extremists, and the rest would be new recruits.

On Saturday, a batch of around 500 fighters marked their completion of the nearly three-week training programme at a ceremony near Syria's northeastern city of Hasakeh.

Dressed in military fatigues, the graduates stood in neat rows and took an oath to protect the country's borders "against all attacks and threats".

Trainers from the SDF and U.S.-led coalition looked on, with pistols strapped to their waists or thighs.

"This is the second (graduating) class of the Border Security Force. They're made up of every demographic in the area," said Kani Ahmad, who headed the training.

The first class graduated on Friday.

The BSF would be deployed from northeast Syria, throughout the Kurdish-controlled north across to the northwestern province of Idlib, he said.

"Their mission is to protect the border, especially threats by Turkey and its mercenaries because we're being threatened," Ahmad added.

Turkey vehemently opposes the creation of the border force because it considers the SDF's Kurdish component -- the People's Protection Units (YPG) -- a "terrorist" group.

The BSF's unveiling prompted an outcry from Ankara, whose escalating threats to attack YPG-held territory on Saturday culminated in Turkish air strikes inside Syria.

The force has also been denounced by Damascus and Tehran, as well as Syria's mainstream opposition.

After completing the 20-day course, graduates would go on to receive more specialised training, officials at the graduation said.

"I'm happy I finished this training," said 21-year-old border guard Jamal Issa, who hails from the town of Kobane near the Turkish border.

"We learned how to use light and heavy weapons, deal with mines and bombs, and first aid," Issa told AFP in Kurdish.

Amer al-Ali, an Arab fighter from the town of Tal Abyad, also along the frontier, said he began fighting alongside the SDF three months ago and was glad to be switching to border monitoring.

"The trainers were from the coalition and had a lot of experience. We learned a lot of tactics on fighting, defence, and attack," he said.

He and his fellow graduates broke out into the Middle Eastern line dance known as the dabkeh and cheered, "Long live the Syrian Democratic Forces!"

Since announcing the plan, the Pentagon has insisted that the BSF is not meant to be an "army" or conventional border force but would primarily seek to prevent a Daesh resurgence in the area.