Mattis met with his Turkish counterpart Nurettin Canikli late Wednesday on the sidelines of a NATO conference in Brussels.

The encounter began with the American appearing stony-faced during the traditional handshake for photographers.

During the talks, Mattis called "for a renewed focus on the campaign to defeat ISIS, and to preventing any vestige of the terrorist organisation from reconstituting in Syria," the Pentagon said in a statement Thursday, referring to the Islamic State group.

Turkey last month launched an operation in northern Syria against a Kurdish militia, the Syrian People's Protection Units (YPG), which Ankara blacklists as a terror group but which is closely allied with the US in the battle against jihadists.

The operation has sparked a festering row between the two countries.

"Mattis acknowledged the legitimate threats posed to Turkey's national security by terrorist organisations," the statement said.

But he "also discussed the complex security environment in Syria, and the danger that a resurgent ISIS could pose to all NATO allies."

It said the two "agreed to continue their full range of bilateral and multilateral defence activities and consultations, and to look for ways to further strengthen defence cooperation in the future."

The meeting came on the eve of a visit to Turkey by US Secretary of State Rex Tillerson.

During his two-day trip, Tillerson -- who last visited in July 2017 -- will hold talks with his Turkish counterpart Mevlut Cavusoglu and President Recep Tayyip Erdogan.