WASHINGTON - The Pentagon has suspended deliveries of equipment related the F-35 fighter jets to Turkey unless Ankara refuses to take a delivery of a Russian missile defence system.

The United States has repeatedly warned Turkey that the Russian S-400 system it aims to buy would compromise the security of the F-35 aircraft, which is made by US company Lockheed Martin.

US officials speaking to Reuters news agency on the condition of anonymity said the next shipment of training equipment and all subsequent shipments of F-35 materials have been cancelled.

"Pending an unequivocal Turkish decision to forgo the delivery of the S-400, deliveries and activities associated with the stand-up of Turkey's F-35 operational capability have been suspended while our dialogue on this important matter continues with Turkey," the spokesperson said on Monday.

Ankara, which is a production partner in the making of the stealth jet as well as a NATO ally, has so far refused to back down on its planned purchase.

The move comes just days after Turkey's Minister of Foreign Affairs Mevlut Cavusoglu said Ankara was committed to a deal to buy the Russian system and was discussing delivery dates.

Last week, four US senators introduced a bipartisan bill that would prohibit the transfer of F-35s to Turkey until the US government certifies that Ankara will not take delivery of the system.(FA)