Turkey has legitimate security concerns along its southern border with Syria, James Mattis, the US defence secretary, said, as Turkish forces continued their offensive against Kurdish fighters in Afrin.

Mattis acknowledged that some members of the Kurdish-led Syrian Democratic Forces (SDF) had shifted to the Afrin region in Syria's north, amid Turkey's ongoing operation against the coalition of armed groups, Anadolu news agency reported.

"The distraction of what's going on up in Afrin right now, which is drawing off some of the [SDF] forces, which have got about 50 percent [of the Afrin region]," Mattis told reporters during his flight to Europe on Sunday, adding that the percentage could be less or more.

The US has long supported the SDF as a "reliable ally" in its fight against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS), despite strong objections by Ankara, which sees the coalition as the Syrian branch of the outlawed Kurdistan Workers' Party (PKK).

About 40,000 people have been killed in Turkey since the PKK launched its fight against the Turkish state in 1980s, seeking more autonomy for the Kurds.

Describing Turkey as the only NATO ally facing an active revolt, Mattis also reiterated that Ankara has real security concerns in the region.

"They [Turkey] have a legitimate security concern, and we do not dismiss one bit of that, along that border with Syria," he stated, adding that Washington will continue to work closely with Turkey.

Turkey says forces aligned to the Kurdish Democratic Union Party (PYD) in Afrin have launched hundreds of cross-border rocket attacks on Turkey, including dozens since late last month, killing and injuring many civilians.

The Pentagon had previously stated that some PYD/PKK-led SDF elements were shifting its fighters, but did not specify where they were headed.

Mattis is expected to meet his Turkish counterpart Nurettin Canikli in Brussels this week.

Turkey launched what it calls "Operation Olive Branch" on January 20 to clear PYD/PKK and ISIL from Afrin.