In a phone call, Trump and Erdogan agree to meet on sidelines of G-20 next month in Japan.

Turkish President Recep Tayyip Erdogan and US President Donald Trump will meet at next month’s Group of 20 (G-20) summit to discuss bilateral issues, including Ankara’s plan to buy a Russian missile defence system that has raised the ire of the United States.

In a tweet on Wednesday, Erdogan’s Communications Director Fahrettin Altun said that in a phone call with Trump, Erdogan reiterated Turkey’s offer for a joint group to overcome an impasse with the US on Turkey’s pledge to buy Russian-made S-400 missiles.

The G-20 summit of leaders from the developed and developing worlds is in Osaka, Japan, from June 28 to 29.

Ties between the NATO allies have been shaky recently over Ankara’s decision to buy the Russian S-400 missile system, which Washington says could compromise its F-35 fighter jets.

US officials have advised Turkey to buy the Patriot missile system rather than the S-400 system from Moscow, arguing it is incompatible with NATO systems.

Turkey has responded, saying that it was the US refusal to sell it Patriots that led it to seek other sellers, adding Russia offered it a better deal, including technology transfers.

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Turkey has suggested that the two countries form a working group that would asses the potential effect of the S-400s on the F-35 aircraft but has yet to hear back from the US.

Washington has threatened to impose sanctions on Turkey if it goes through with the purchase of the Russian missile system, a move that would likely damage Turkey’s economy. The lira has already declined about 14 percent this year in part due to concerns over the potential sanctions.

In Wednesday’s call, Erdogan also welcomed the US decision this month to drop additional tariffs on Turkish steel. Trump had doubled the tariffs to 50 percent in August 2018 amid a diplomatic spat over the imprisonment of an American pastor. That pastor, Andrew Brunson, was released in October 2018.

Separately on Wednesday, Turkey released Serkan Golge, a NASA scientist with dual US-Turkish citizenship whose nearly three-year detention has also soured relations between the NATO allies, the US State Department said.

Golge was detained in July 2016 on a visit to his country of birth as Erdogan cracked down on alleged supporters of self-exiled Muslim preacher Fethullah Gulen, whom Turkey accused of orchestrating a failed coup.