"About 100 military servicemen from the Chinese People's Liberation Army who will maintain the second S-400 regiment set, completed a training course to use the S-400 systems in July and returned to their home country," the source said, adding that the training had been launched in March 2019.

MOSCOW, July 31. /TASS/. About 100 military servicemen from the Chinese People's Liberation Army who will maintain the second regiment set of the S-400 missile systems have completed a training course in Russia to use the systems and returned to China, a military diplomat told TASS on Wednesday.

The source also pointed out that the training course had coincided with the start of the delivery of the second S-400 regiment set to China.

Russia’s Federal Service for Military-Technical Cooperation declined to comment on the information.

Another military diplomat told TASS on July 24 that Russia had launched an operation to deliver the second regiment set of S-400 missile systems to China by sea. At least three vessels are planned to be involved in the operation.

China was the first country to purchase the S-400 systems. Moscow and Beijing singed a contract for the delivery of two S-400 regiment sets in 2014. The first set was delivered to China in the spring of 2018. The systems were successfully tested.

The S-400 Triumf (NATO reporting name: SA-21 Growler) is the most advanced long-range anti-aircraft missile system that went into service in 2007. It is designed to destroy aircraft, cruise and ballistic missiles, including medium-range ones, and can also be used against ground objectives. The S-400 system can engage targets at a distance of 400 km and at an altitude of up to 35 km.