Turkey has started testing the first S-400 missile defence system it purchased from Russia, Milliyet newspaper reported on Monday.

The Turkish military is testing the radar connection between its aircraft and the S-400 surface-to-air missile system in trials in Ankara, Milliyet said. The tests started after the Ankara Governor’s Office announced that F-16 fighter jets would make flights over Turkey’s capital in the next few days.

The testing, which is designed to ensure that the S-400s can identify Turkish jets, is being carried out from the former Akıncı Airbase, renamed the Mürted (Apostate) Airbase after it was used in a failed military coup in 2016, Milliyet said, citing officials in Turkey’s defence procurement agency. The system will be fully operational in April 2020. Delivery of a second S-400 system is expected to start at the end of next year, the government has said.

Turkey’s decision to acquire S-400s has soured relations with its NATO allies. The United States halted delivery of 100 F-35 stealth fighter jets and suspended Turkey’s participation in the programme to build them after it started receiving the S-400 batteries in July. Ankara also faces possible U.S. economic sanctions in response to the decision.