U.S. President Donald Trump has met with his Turkish counterpart, Recep Tayyip Erdogan, to urge him not to proceed with the purchase of Russia’s advanced S-400 ground-to-air defense system.

When the two leaders met on the sidelines of the G20 summit in Osaka, Japan, on June 29, Trump told Erdogan that the proposed purchase was “a problem.”

“It’s not good,” Trump said. “It’s not good.”

Trump also described this issue as “complicated.”

Washington has said that if fellow NATO member Turkey does not cancel the S-400 deal by July 31, Ankara will be blocked from purchasing the next-generation F-35 fighter. The United States has urged Turkey to purchase the U.S.-made Patriot missile system instead.

Earlier, however, Erdogan met with Russian President Vladimir Putin, telling the Russian leader that the S-400 deal was “of prime importance.” He gave no indication that Ankara was reconsidering the purchase.

During his meeting with Trump, Erdogan said “our strategic partnership requires solidarity in many fields.”

“I strongly believe such solidarity will continue in the upcoming period,” he added.

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