A leading Democrat on the U.S. Senate Foreign Relations committee says Turkey’s recent purchase of antiaircraft missile systems from Russia may have violated a U.S. law that requires automatic sanctions to be imposed against Ankara.

Senator Ben Cardin issued the warning on September 14 in a letter to the administration of President Donald Trump.

The letter says Ankara’s purchase of Russian S-400 surface-to-air missiles, which was finalized on September 12, violates U.S. congressional sanctions against Russia that were signed into law in August.

Cardin noted that the legislation calls for sanctions “on any person that conducts a significant transaction” with Russia’s defense or intelligence sectors.

“These are mandatory sanctions and constitute a commitment by the United States to deter Russia from attacking the United States and its allies in the future,” the letter said.

Cardin wrote: “As a U.S. ally, it is unfortunate that Turkey has appeared to align itself with Moscow during this critical time.”

He also asked the U.S. State Department to assess how the deal might affect Turkey’s NATO membership and U.S. security assistance to Ankara, which includes weapons sales.

Turkish President Recep Tayyip Erdogan has dismissed Western concerns about the $2.5 billion deal, saying NATO only has itself to blame because it has been slow to provide his country with weapons systems.

Erdogan’s spokesman, Ibrahim Kalin, said on September 14 that good relations between Ankara and Moscow do not represent a break with the West, despite what he said was the “discomfort” expressed by some NATO allies.

Based on reporting by Politico.com, Reuters, and dpa