US officials mis-spoke about a plan to set up a 30,000-strong militia in eastern Syria and owe angry ally Turkey an explanation, Secretary of State Rex Tillerson admits.

On Sunday, the US-led coalition fighting the Islamic State group in Syria announced that it was training local fighters, including Kurdish militia, as a "border security force."

Turkey, which regards the US-backed YPG militia as a faction of the outlawed Kurdish separatist PKK and thus a terrorist group, reacted with fury and vowed to destroy the new unit.

Pentagon officials have since backtracked on how they describe the force, insisting it will operate within Syria to protect areas liberated from the Islamic State group.

But Turkey has not been reassured and Tillerson, who met his Turkish counterpart Foreign Minister Mevlut Cavusoglu on Tuesday in Vancouver, admits the issue was badly handled.

"It's unfortunate that entire situation has been mis-portrayed, mis-described. Some people mis-spoke," he told reporters on his plane late Wednesday.

"We are not creating a border security force at all," he said, adding that he had spoken to US Defense Secretary Jim Mattis to confirm this.

"We have shared with the Turks what we are doing is we are trying to ensure that local elements are providing security to liberated areas," he explained.

In Ankara, even after talking to Tillerson, Cavusoglu said Turkey continues to reject the idea of the force.

"Did this satisfy us in full? No, it did not," he told CNN-Turk television.

"The establishment of a so-called terror army would cause irreversible damage in our relations ... it is a very serious situation," he warned.

Tillerson explained that the Islamic State group, while diminished, is still capable of carrying out attacks in parts of northwest Syria and in the Euphrates valley.

The new US-backed force "is just more training and trying to block ISIS from their escape routes" he said, not a means to protect Kurdish areas on the Turkish border.

"I think it's unfortunate that comments were made by some that left that impression. That is not what we're doing," he said.

"We owe them an explanation. It was not properly described, and it's unfortunate. We understand why they reacted the way they did."