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ISTANBUL, April 10 (Reuters) – President Tayyip Erdogan said the delivery of Russian S-400 missile defence systems may be brought forward from July, the Sabah newspaper reported on Wednesday, sticking to a purchase which has put Turkey at odds with NATO ally the United States.

The S-400s are not compatible with NATO systems and Washington says Turkey’s purchase of them would compromise the security of F-35 fighter jets, which are built by Lockheed Martin Corp. Turkey is involved in F-35 production.

Erdogan told reporters on his plane while returning from a trip to Russia this week that he and his foreign minister were constantly being asked about the S-400 purchase and being pressured to abandon it.

“We answer that ‘this deal is