WASHINGTON - The United States and Turkey plan to conduct combined patrols in northern Syria to enhance security there, US Chairman of the Joint Chiefs of Staff Joseph Dunford said on Tuesday.

Speaking at a press briefing, Dunford said that the two nations began working together about two months ago on the security in northern Syria with a focus on Manbij area.

The patrols will be conducted in two phases.

"One is independent patrols with coordination and communication between Turkish forces, US and coalition forces. The second phase is combined patrols," he said.

"We are now conducting the independent patrols with communication between the Turks, and we're still planning, with the Turks, for combined patrols," he added.

In order to do combined patrols, both sides need a "command and control construct set-up," joint training in Turkey and an agreement "on rules of engagement and other details of patrolling," Dunford noted.

The bilateral conversation in these respects is ongoing, he said. "It's ongoing. We're both satisfied, that is the Turks and the United States, with the pace of our planning ...We're talking about making some refinements to that security in the future."

Also at the press briefing, US Defense Secretary James Mattis said Turkey's purchase of Russia-made S-400 system does "concern" the US side.

"Clearly Turkey is bringing a Russian anti-aircraft, anti-missile system into a NATO country. We cannot integrate that into NATO. Yes, it does concern us, and we do not recommend that," he added.

The move came in line with the roadmap on Manbij agreed to by Turkey and the United States earlier this year, which focused on the withdrawal of the People's Protection Units (YPG) militants from northern Syria.

Turkey regards the YPG as the Syrian branch of the Kurdistan Workers' Party (PKK), and has long been urging the United States to remove the YPG from Manbij, where about 2,000 US soldiers are currently deployed.

Relations between Turkey and the United States have been strained over Washington's support for the YPG.