Moscow has already delivered two shipments of its top air defences to Turkey, despite efforts by the US to disrupt the deal between the two countries. In a bid to prevent it, Washington froze F-35 shipments to Ankara, forcing it to consider buying fifth-generation fighters elsewhere, including from Russia.

The Turkish Defence Ministry has announced that its troops have started training to operate the S-400 air defence systems, the deliveries of which commenced in July 2019. According to the statement, the training is taking place in the Russian city of Gatchina.

"S-400 training has started in Gatchina, Russia with the participation of Air Force Command personnel as part of a long-range air-and-missile defence system project", the ministry's official Twitter account said.

Earlier in the day, the head of Turkish Defence Industries, Ismail Demir, revealed that the top-of-the-line Russian defence systems would be operational by April 2020, meaning that they will be shipped in full and that the soldiers will have finished their training by that time.

Turkish Foreign Minister Mevlut Cavusoglu has earlier suggested that the country could order additional S-400s if the US again refuses to sell its Patriot missile systems, something that Washington has offered Ankara as an alternative to the Russian weaponry.

By offering their systems, the US hoped to convince Turkey to ditch the S-400 purchase from Russia, something that Washington has insisted on since the deal was inked in December 2017. While Ankara has expressed interest in the offer, it has refused to ditch the S-400s, calling them crucial for the country's national security.

Washington then employed another tactic, going through with its longstanding promise to suspend shipments of F-35 jets, the development of which was partially sponsored by Turkey. The US claims that the S-400s could pose a threat to the fighters, potentially revealing their weaknesses to Moscow. Ankara has suggested creating a working group to iron out these concerns, but the US has ignored the proposal.

Following the US decision regarding the F-35s, Ankara announced that it could look for a replacement elsewhere, recently even setting its gaze on Russia's fifth-generation Su-57 fighter jet. While Moscow has expressed its readiness to sell Su-35s, Russian President Putin stated during the MAKS-2019 air show that selling Su-57s is also a possibility, as is joint production of either of the jets.