Turkish army tanks are seen in Afrin, Syria, on Jan. 22, 2018. (Xinhua)

WASHINGTON, Jan. 22 (Xinhua) -- The United States is "very alert" to the ongoing situation about Turkey's offensive against U.S.-backed Kurdish militia in northern Syria, said U.S. Defense Secretary James Mattis on Monday.

"We are very alert to it. Our top levels are engaged, not just in Defense Department," Mattis told reporters while travelling to Asia, according to a statement from Pentagon.

Turkey on Saturday launched an operation dubbed "Operation Olive Branch" with intense air raids and shelling seeking to oust from the Afrin region of northern Syria the Peoples' Protection Units (YPG), which Ankara considers as a terror group affiliated to the Kurdistan Workers' Party (PKK).

"They (Turkish authorities) warned us before they launched the aircraft they were going to do it," said Mattis en route to Indonesia and Vietnam.

Ankara' s military operation came after U.S. officials' statements on a planned Kurdish-led "border security force (BSF)" to be established in the future in Syria along the 900-km border with Turkey, a move that has angered Turkey.

The BSF would be mainly constituted by the YPG, supported politically and militarily by the United States since 2014 in the battle against the Islamic State.

Turkish President Recep Tayyip Erdogan declared earlier that he would "kill the force before it is even born."

"Turkey has legitimate security concerns," Mattis told reporters, adding that the United States and Turkey were "working now on the way ahead" through diplomatic and military channels. "We'll work this out," the Pentagon chief said.

Meanwhile, Erdogan said Monday that Turkey will not take a step back from its operation against YPG in Afrin.