Turkey has received the first parts of a Russian S-400 missile defence system despite opposition from the US.
The shipment arrived in an airbase in the capital Ankara on Friday, the Turkish defence ministry says.
The move will anger the US, which has warned that Turkey cannot have both the S-400 anti-aircraft defence system and US F-35 fighter jets.
Turkey and the US are Nato allies - but Turkey has also been establishing closer links with Russia.
Turkey has signed up to buying 100 US F-35s, and has invested heavily in the F-35 programme.
However, US defence officials have said they do not want the F-35 jets to be near S-400 systems - because they fear Russian technicians will be able to access the F-35's vulnerabilities.
This looks set to prompt a major rift between Washington and one of its key Nato allies. For an alliance member to buy this kind of equipment from Russia is almost unprecedented.
The US has already halted deliveries of the aircraft to Turkey and suspended the training of Turkish pilots.
Plans are also under way to remove Turkey from the programme altogether. It manufactures part of the F-35 and is due to be a regional hub for maintenance of the aircraft.
Turkish President Recep Tayyip Erdogan seems to believe that, whatever the Pentagon may say, Donald Trump himself is less hostile to the purchase of the Russian missiles.
A major test of ties between Ankara and Washington beckons.
How does the S-400 work?
1. Long-range surveillance radar tracks objects and relays information to command vehicle, which assesses potential targets
2. Target is identified and command vehicle orders missile launch
3. Launch data are sent to the best placed launch vehicle and it releases surface-to-air missiles
4. Engagement radar helps guide missiles towards target
BDST: 1619 HRS, JULY 12, 2019