A Russian serviceman walks past S-400 missile air defence systems in Tverskaya Street before a rehearsal for the Victory Day parade, which marks the anniversary of the victory over Nazi Germany in World War Two, in central Moscow, Russia April 29, 2019. Photo: REUTERS/Tatyana Makeyeva.

Turkey’s defense ministry said on Sunday that the delivery of a second battery of Russian S-400 missile defense systems has been completed as of Sunday, and added that the systems would become active in April 2020.

Ankara and Washington have been at loggerheads over Turkey’s purchase of the S-400 systems, which the United States says are not compatible with NATO defenses and pose a threat to Lockheed Martin’s F-35 “stealth” fighter jets.

The initial parts of the system were delivered to Ankara in July despite warnings about possible US sanctions over the purchase. The United States has also expelled Turkey from the F-35 program, but Ankara has so far dismissed the warnings.

In a statement, the defense ministry said the delivery of the second S-400 battery to Ankara was completed. Efforts to mount the systems and train personnel who will use them were continuing, it said, adding that it planned to activate the S-400s in April 2020.

Turkish Foreign Minister Mevlut Cavusglu told an interview with CNN Turk on Saturday that the S-400s would be activated despite repeated US warnings.

“They (US officials) told us ‘don’t activate them and we can sort this out,’ but we told them that we didn’t buy these systems as a prop,” Cavusoglu said, adding that Turkey would be open to buying US company Raytheon’s Patriot systems as well.

In an interview on Friday, Turkish President Tayyip Erdogan told Reuters he will discuss buying Patriot missiles with President Donald Trump this month, saying his personal bond with the US leader could overcome the crisis caused by the S-400s.

Though Washington has not yet announced whether it will impose sanctions on Ankara, Trump has shown sympathy towards Turkey. He has not fully ruled out sanctions.

On Monday, US Treasury Secretary Steven Mnuchin said the Trump administration was considering imposing sanctions related to Turkey’s purchase of the S-400 systems, but no decisions have been made.

The dispute over the S-400 systems is one of several issues straining ties between the United States and Turkey that include the ongoing conflict in Syria, among others.