"Clearly we continue to talk to the Turks about the possibility of a secure zone, whatever you want to call it," Lieutenant General Kenneth McKenzie told reporters.

"We've looked at that for a couple of years in various different iterations and no final decision on it yet. Our military commanders are still talking so I would say it's a concept that's out there ... it's simply an idea that's floating around right now."

McKenzie did not go into details about what a safe zone might entail, but Turkish media have said Secretary of State Rex Tillerson told his Turkish counterpart Foreign Minister Mevlut Cavusoglu that he supports creating a secure area that reaches 30 kilometers (18.5 miles) into Syria.

The brutal Syrian war, which has claimed more than 340,000 lives since 2011, has grown even more complex over the past week with Turkey launching a ground operation against Kurdish fighters in northern Afrin, close to the Turkish border.

That has heightened tensions with NATO member Turkey's Western allies -- particularly the United States, which has backed the Kurdish YPG in their fight against the Islamic State group.

"Particularly in Afrin, Turkish operations ... that have the effect of inducing friction into the equation, of making it harder to focus on why we are in Syria ... are a negative thing," McKenzie said, adding that he understood Turkey's "legitimate" security concerns.

The three-star general added that the Turkish operations in Afrin were "not helpful" in the fight against IS.

He would not comment on what the US military would do if the Turks move to the east and continue their push against the Kurds to the Kurdish-held city of Manbij.

If that happens, America would find itself at an extraordinary crossroads.

It would need to determine whether it would support its NATO ally Turkey, the Kurdish fighters it has backed to defeat IS, or somehow forge some sort of compromise between the two sides.