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

“Our Honorable 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.

Washington has said the missile system 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 two leaders exchanged views on recent developments in Syria and agreed to cooperate closely in the fight against terrorism, the Turkish presidency said in a statement. Erdoğan and Trump also reaffirmed their commitment to increase bilateral trade between both countries to $75 billion annually, and discussed steps to be taken toward reaching that goal.

The presidency said Erdoğan once again proposed forming a joint technical group regarding Turkey’s pending procurement of the S-400 defense system from Russia, which has become a major point of contention between Washington and Ankara. Tensions between the U.S. and Turkey reached a fever pitch in recent months with Turkey set to begin receiving the advanced S-400 Russian surface-to-air missile system which the U.S. says will jeopardize Turkey’s role in the F-35 fighter jet program and which could trigger congressional sanctions.

Turkey decided in 2017 to purchase the S-400 system following protracted efforts to purchase air defense systems from the U.S. with no success, and the first S-400 is likely to be delivered in June. The presidency added that Erdoğan condemned the armed assault in San Diego and offered his condolences to Trump and the American people. A gunman walked into a suburban San Diego synagogue on Saturday morning, the last day of the weeklong Jewish holiday of Passover, and opened fire with an assault-style rifle, killing one woman and wounding three others.