According to the Countering America’s Adversaries Through Sanctions Act, which Trump signed into law last month, the administration is required to sanction any foreign entities that engage in significant transactions with the Russian Federation’s defense and intelligence sectors, according to lawmakers and experts.

Erdogan announced last week he signed the $2.5 billion contract with Russia and put down a deposit, ignoring public and private U.S. warnings. He defended the move Tuesday in an interview with PBS News Hour, promising to see it through.

“We have asked for those weapons from many NATO allies and primarily the United States, but we were turned down. That’s why we have to resort to other means,” he said. “Russia is willing to support us all the way to a possible joint manufacturing of these missiles.”