Turkey gave Washington advance warning before launching an operation against US-allied Syrian Kurdish forces, and Ankara has "legitimate" security concerns in the area, Defense Secretary Jim Mattis said Sunday.

"Turkey was candid. They warned us before they launched the aircraft they were going to do it in consultation with us, and we are working now on the way ahead through the ministry of foreign affairs," Mattis told reporters aboard his aircraft at the start of a trip to Asia.

Turkey "is the only NATO country with an active insurgency inside its borders, and Turkey has legitimate security concerns," Mattis said, referring to the Kurdistan Workers' Party (PKK), which has been engaged in a separatist struggle against Ankara since 1984.

Turkey launched "Operation Olive Branch," an offensive by Ankara's troops and allied Syrian rebels against the Kurdish People's Protection Units (YPG) in the town of Afrin.

The YPG has been a key US ally in the war against the Islamic State group, helping to drive the group's jihadist fighters from swaths of Syrian territory, including its stronghold Raqa.

But Ankara considers YPG fighters to be "terrorists" linked to the PKK.

- Call for 'restraint' -

Turkey on Sunday ruled out the risk of an inadvertent clash with American forces in its operation in Syria, saying there were no US troops in the area where the campaign was taking place.

"US officials declared that there has been no American military or American soldiers in the region," Turkish Deputy Prime Minister Bekir Bozdag told reporters in Istanbul.

The US State Department, while also referring to the "legitimate security concerns of Turkey," issued a call for Ankara to proceed carefully.

"We urge Turkey to exercise restraint and ensure that its military operations remain limited in scope and duration and scrupulous to avoid civilian casualties," State Department spokeswoman Heather Nauert said in a statement.

"We call on all parties to remain focused on the central goal of defeating" IS, Nauert said.

A Britain-based monitoring group and a YPG spokesman both said that Turkish air raids on Sunday killed eight civilians in northern Syria.

A day earlier, the YPG's Birusk Hasakeh told AFP that a Turkish bombardment had killed 10 people, including seven civilians.

Turkish Foreign Minister Mevlut Cavusoglu said Sunday that claims of civilian casualties from the offensive were untrue.