US President Donald Trump speaks with Turkish President Recep Tayyip Erdogan ahead of the opening ceremony of a NATO summit in Brussels, Belgium, July 11, 2018. Photo: Ludovic Marin / Pool via Reuters.

US President Donald Trump and Turkish President Recep Tayyip Erdogan in a phone call on Monday discussed a Turkish proposal to create a joint working group on its planned purchase of a Russian S-400 missile defense system, the Turkish Presidency said.

“Our Honourable President brought up the proposal to establish a working group regarding the procurement of the S-400 defense system from the Russian Federation,” it said.

The two countries have argued for months over Turkey’s order for the missile defense system, which the United States says is incompatible with NATO’s defense network and could compromise its F-35 fighter jets.

Ankara has said a working group could assess the impact of the S-400 on the fighter jets, but on Friday said that it has not heard back from the Americans.

The White House confirmed on Monday that Trump and Erdogan had discussed Turkey’s planned purchase of the S-400 missile defense system.

Asked if Trump raised with Erdogan the US concerns about Turkey buying the S-400 system, a senior administration official said:

“We have been clear and consistent in emphasizing our grave concerns on the S-400 acquisition with representatives of the Turkish government on numerous occasions and at the highest levels.”

US Secretary of State Mike Pompeo and several US senators have warned Turkey it will face penalties for buying S-400s under a law that calls for sanctions against countries procuring military equipment from Russia.

Turkey has said that as a NATO member, it poses no threat to the United States and the sanctions should not apply.

Trump and Erdogan also discussed trade and the ongoing crisis in Syria, the White House said.