Turkish President Recep Tayyip Erdogan said on Monday that the Russian S-400 missile defense system, parts of which have been delivered to Turkey over the past four days, will be fully deployed in April 2020.

Turkey’s purchase of the Russian system has raised tensions with its NATO allies, particularly the United States, which has warned Turkey that it will respond with sanctions.

Speaking at Istanbul’s Ataturk Airport on the third anniversary of 2016’s attempted coup, Erdogan said eight planes had already brought parts of the Russian system and more are coming, as he had repeatedly promised.

"With God’s permission, they will have been installed in their sites by April 2020," he told the crowd.

"The S-400s are the strongest defense system against those who want to attack our country. God willing, we are doing this as a joint investment with Russia, and will continue to do so."

U.S. officials have said that in addition to being hit with legislation aimed at preventing countries from purchasing military equipment from Russia, known as CAATSA, Turkey could be thrown off the F-35 stealth fighter jet program.

That would mean it would no longer make F-35 parts or be able to buy the jets it has ordered.

On Sunday, Erdogan said that U.S. President Donald Trump has the authority to waive sanctions on Turkey for its purchase of Russian air defense systems and should find a "middle ground" in the dispute.