"Delivery of S-400 Long Range Air and Missile Defence Systems resumed today," the statement said.

"The fourth Russian plane carrying S-400 parts landed at Murted Airport outside Ankara," it added.

The US fears that if Ankara integrates the S-400 into its defences, data about the US-built F-35 fighter jet could leak back to the Russians, and Washington has threatened to deny Turkey access to the stealth aircraft.

The western defence alliance NATO, which counts Turkey as one of its members, is also "concerned about the potential consequences" of the S-400 purchase, an official told AFP.

Members of the US Congress have repeatedly voiced opposition to the move and threatened sanctions.

"President Erdogan was given a very clear choice. Unfortunately, he has clearly made the wrong one," said Eliot Engel and Michael McFaul, the top Democrat and Republican respectively on the House Foreign Affairs Committee.

Ankara rejects the US warnings, and on Friday, Foreign Minister Mevlut Cavusoglu told reporters: "We say this each time. This is a done deal. The process continues."