Russia has begun delivering the second batch of S-400 air and missile defense systems to China by sea, a source has revealed.

"A transport vessel with the S-400s meant for China has taken to the sea from the port of Ust-Luga on the Baltic coast. Therefore, like in the case with Turkey, the deliveries of the systems have started several months ahead of the term stipulated in the contract," a source from the military was quoted as saying by TASS on Wednesday.

The source added that not more than three vessels would be required to complete the delivery process. The vessels will take to the sea one by one within short time intervals, he said.

China became Russia’s first international customer for the air defense systems in 2014 when Beijing reportedly inked a $3 billion deal for four to six units.