(CNN) Turkey warned the United States before carrying out airstrikes against U.S.-backed Kurdish militia in Syria's Afrin province, Defense Secretary James Mattis said Sunday, according to a report by Reuters.

"Turkey was candid," the defense secretary said on a flight to Asia, the Reuters report said. "They warned us before they launched the aircraft they were going to do it, in consultation with us. And we are working now on the way ahead. We'll work this out."

Turkey's military told Washington of the air operation in a phone call to high level US military officials, Reuters reported. No US forces were at risk from the Turkish military operations, said Mattis, who did not disclose the US reaction to Turkey's warning, Reuters said.

On Sunday, Turkey launched a ground operation across the border into northern Syria, in a move likely to raise tensions with the United States.

Turkish officials said the troops entered the Afrin area a day after Turkish jets pounded targets there in an attempt to drive US-allied Kurdish militia from the area. "Operation Olive Branch is ongoing as planned and the ground operation has started," the Turkish armed forces said in a statement.