President Donald Trump on Sunday suggested that Canada could face some form of penalty from the United States over smoke that’s come across the border from the nation’s massive wildfires.

A reporter on Air Force One asked the president if he had decided on a “retaliation” for Canada over wildfire smoke, as he had threatened last month.

Wildfire smoke from Canada shrouds the sun as it rises behind the skyline of lower Manhattan and One World Trade Center in New York City on July 18.

Gary Hershorn/Getty Images

It’s not clear yet what form that penalty may take ― but it will likely be on top of the 50% tariff Trump announced just last month on most Canadian goods. The president said it was in retaliation for what he called discrimination against U.S. cars, booze and dairy products.