Escorted by two guards, he quickly went into the VIP section, while four others who appeared to be accompanying Kim entered the boarding gate.

The Russian Aeroflot SU 1700 flight carrying Kim took off at 4:10 p.m. and was expected to land in Vladivostok early Sunday.

The visit by Kim, known as leader Kim Jong-un's de facto chief of staff, raised speculation that Pyongyang could be making preparations for leader Kim's first trip to Russia since taking office.

A vehicle possibly carrying Kim Chang-son was seen coming into the Kremlin every day from Wednesday to Friday, suggesting the official held talks with Russian counterparts over logistics and protocol.

Since Kim Chang-son is bound for the Russian port city, it may be that leader Kim is considering either Moscow or Vladivostok as the venue.

Ahead of Kim's visits to Singapore in June last year and Vietnam last month for his summit meetings with US President Donald Trump, Kim Chang-son visited the two cities well ahead of time to make logistical and protocol preparations.

Since the breakdown of last month's summit between Kim and Trump, North Korea has been stepping up its diplomacy with Russia, with three senior North Korean officials making back-to-back visits to Russia this month.

A delegation of Russian senators also visited Pyongyang this week. (Yonhap)