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.

"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.

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.