Cargo planes carrying the first of the Russian S-400 missile defence systems Ankara has purchased from Moscow have departed a Russian airbase and are set to arrive in Turkey on Monday or Tuesday, Habertürk reported.

U.S. officials have repeatedly told NATO-ally Turkey that the S-400 poses a threat to U.S. military systems, particularly the F-35 stealth fighter jet, and warned that sanctions would be triggered by Turkey’s installation of the Russian system. A Turkish presidential spokesman said on Thursday that the S-400 posed no threat and would soon be deployed.

The initial S-400 delivery has been sent on two cargo planes from a Russian military base, said Habertürk, and a Russian technical team is set to arrive in Turkey on Monday to oversee installation.

Sanctioning Turkey and expelling it from the F-35 program, as the United States has vowed to do, would mark one of the most significant ruptures in U.S.-Turkey relations.