President Tayyip Erdoğan said on Monday that the Russian S-400 missile defense system, parts of which have been delivered to Turkey over the past four days, would 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 Atatürk Airport on the third anniversary of 2016's attempted coup, Erdoğan said eight planes had already brought parts of the Russian system and more were 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. We are doing this as a joint investment with Russia, and will continue to do so," he said, expressing intent to jointly produce the systems with Moscow.

The Ankara-Moscow S-400 deal was inked in December 2017, when the parties signed a $2.5 billion agreement for two batteries of the S-400, which is Russia's most advanced long-range anti-aircraft missile system in use since 2007.

The 2017 contract with Russia followed lengthy efforts to purchase an air defense system from the U.S. with no success.

In recent months U.S. officials urged Turkey to buy U.S. Patriot missiles, arguing the Russian system would be incompatible with NATO systems and expose U.S. F-35 fighter jets to possible Russian subterfuge.

Turkey, however, emphasized the S-400 would not be integrated into NATO systems and would not pose a threat to the alliance.

Turkey has urged the formation of a commission to clarify any technical issues, but the U.S. has failed to respond to this proposal.

The U.S. has also threatened sanctions over the S-400 purchase, with Turkey responding that any sanctions would be met in kind.