Turkish President Recep Tayyip Erdogan has said that Russian air-defense missile systems could be delivered earlier than initially planned, standing by a purchase that has put the NATO member at odds with Washington.

'The delivery of the S-400 missile-defense system was to be in July, maybe it can be brought forward,' Erdogan told journalists on his plane following a trip to Russia, Turkish media reported on April 10.

The United States has demanded that Ankara call off its deal with Russia, saying that the S-400 missiles are incompatible with NATO systems and are seen as a threat to U.S.-made F-35 fighter jets.

On April 1, Washington said it was halting deliveries to Turkey related to the F-35 program in response to Ankara's decision to move ahead with the purchase of the Russian air-defense systems.

Washington has said it could also withdraw an offer to sell Ankara the U.S. equivalent -- the Patriot antimissile system.

Russia media reported that Turkey intended to buy four S-400 units for a price of $2.5 billion.

In a television interview on April 10, Turkish Foreign Minister Mevlut Cavusoglu said that his country could look 'elsewhere' if the United States doesn't deliver F-35 fighter jets.

Cavusoglu said that Turkey could also consider acquiring more S-400s or other systems if it can't purchase the Patriot system.

'If the United States does not want to sell the Patriot, tomorrow we can buy a second S-400, or another defense system,' he told the broadcaster NTV.

In Moscow, Kremlin spokesman Dmitry Peskov said Russia was 'open' to a second purchase.

Turkey's relations with Russia have historically been tense, but Moscow and Ankara have established strong economic ties since the end of the Cold War.

Under Erdogan and Russian President Vladimir Putin, the countries have moved closer in recent years amid severe tension between Russia and the West and strains in Turkey's ties with the United States and European Union.

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