"Delivery of S-400 Long Range Air and Missile Defence Systems resumed today," a statement by the Turkish defence ministry said, adding that the fourth Russian plane carrying S-400 parts had landed at Murted Airport outside Ankara.
Turkey has opted for the Russian S-400 “Triumph” advanced aerial defence system, considered by experts to be equal if not superior to the American “Patriot” system, but cheaper.
The purchase has sparked a row within Nato, with the US threatening sanctions against Turkey for buying the Russian-made missiles.
The US fears that if Ankara integrates the S-400 into its defence network, data about the US-made F-35 fighter jet could leak back to the Russians.
Earlier this year, the US suspended deliveries related to the F-35 and gave Turkey until the end of July to cancel the S-400 deal, failing which Turkish pilots learning to fly F-35 in the US would be expelled.
The Nato defiance alliance finds itself in an awkward position, torn between concern over the defence system of one of its members and the need to avoid putting Turkey on the spot.
It was therefore on condition of anonymity that one Nato official told AFP he was “worried about the consequences of the Turkish decision to own the S-400”, noting that this defence system contradicts those used by the alliance.
'Trojan horse' fears
The "consequences" could relate to the principle of interoperability whereby member states are free to decide on their military equipment but it must be compatible, interoperable, with other member states. This is considered not to be the case with the Russian-made missiles.
The real concern, however, is that integrating Russian technology into a Nato member's defence system could render it – and ultimately other members' systems – vulnerable.
In Brussels, some are talking of a potential “Trojan horse” within Nato, says RFI's Brussels correspondent Pierre Bénazet.
A new rapprochement between Moscow and Ankara
Relations between Moscow and Ankara were soured in 2015 when Turkey shot down a Russian plane close to the Syrian border.
But since the failed coup against President Recep Tayyip Erdogan’s government in 2016, relations have warmed and the sale of the S-400 has cemented the new friendship, much to Moscow’s satisfaction.
In addition to the lucrative deal, estimated at around $2 billion, Russia “has come out on top at every level,” according to RFI’s Moscow correspondent Daniel Vallot. “Moscow is getting closer to Ankara and pulling Turkey further away from the US. This is very important because Russia has made Turkey a key player in its Middle East policy, notably on the Syria question.”
The shift towards Russia and away from the US also corresponds to popular opinion in Turkey, says Sinan Ulgen, head of the Edam think tank, not least because of Fethullah Gulen whom the Turkish government holds responsible for the failed coup.
“The Turkish government, but also many Turkish people, see the USA as an unreliable partner,” he told RFI. “Washington’s support for the Kurds in Syria, its lack of empathy for the case of Fethullah Gulen who lives in the United States.
"There’s a feeling that the US is not on same wavelength as its ally, Turkey, in helping it with the questions of national security. ”