Russia moves more S-400 systems near North Korea border.

Russian Ministry of Defense
Russia sent more S-400 anti aircraft systems to Vladivostok, a region which is located around 130km away from North Korea border. This move has come among increasing tensions between North Korea and the United States exacerbated by increasing rhetoric between US president, Donald Trump, and North Korean leader Kim Jong-Un.
The S-400 Triumf SAM system also known by NATO as SA-21 Growler, was developed in the 1990s as an upgrade to the S-300 systems. It utilizes 4 missiles which vary from the very long range 40N6 missile (400km) to the short range 9M96E2 missile (40km). Each system costs $400 million and around 69 have been produced so far. Source: CSIS.