In a sign of rising tensions between the U.S. and Turkey, the United States has halted delivery of gear related to its new F-35 fighter jet ..

Making good on a threat to withhold the advanced jet if Turkey goes ahead with plans to buy a Russian missile defense system.

Reuters has learned the U.S. has informed Ankara it will not be getting any more equipment needed to fly the radar-evading jet unless it drops plans to use Russia's S-400 air defense system.

The US says the Russian system would compromise the security of the F-35, but so far Turkish President Tayyip Ergogan has refused to back down.

The U.S. and NATO allies who fly the F-35 fear Russia could use the radar on its missile system to learn how to spot and track the F-35s - compromising the security of the jets and making them more vulnerable to Russian weapons.

Turkey makes parts of the fuselage, landing gear and cockpit displays of the Lockheed Martin-made jets, but U.S. sources tell Reuters the Turkish elements can be replaced.

Turkish pilots are training on two of the $90 million F-35s at an airbase in Arizona, but the country was not due to take delivery of it first planes until November.

Analysts say removing Turkey from the F-35 program will open a wider rift amid differences over Syria, Iran sanctions, and Turkish demands that the US extradite dissident Islamic cleric Fethullah Gulen.