Defense Minister Shin Won-sik speaks during a meeting at the Ministry of National Defense in Yongsan District, Seoul, in this picture provided by the Ministry of National Defense, July 17. Yonhap

North Korea is believed to have sent containers that can carry more than 5 million artillery shells to Russia, South Korea's defense chief has said, amid persistent concerns over deepening military cooperation between the two nations.

Among shipments presumed to have been delivered to Russia from North Korea as of July 15 were containers that can hold some 5.2 million artillery shells as well as scores of short-range ballistic missiles, Defense Minister Shin Won-sik said in an interview published in the Japanese Yomiuri Shimbun newspaper earlier in the day.

Shin reaffirmed South Korea's stance that its possible provision of arms supply to Ukraine would hinge on the level and details of military cooperation between Moscow and Pyongyang, such as transactions of weapons and military technology.

Noting how Kim Yo-jong, the powerful sister of North Korean leader Kim Jong-un, threatened of "gruesome and dear" consequences against anti-Pyongyang leafleting by activists groups in South Korea and hinted at a change in its response, Shin said the North may attempt to strike locations where such leaflets are launched.

"We are closely monitoring the situation as there is a possibility of (North Korea) shooting down leaflet-carrying balloons sent from the South, or shooting or shelling locations where the balloons are floated," Shin was quoted as saying.

In 2014, the two Koreas exchanged machine gun fire across the border after the North apparently tried to shoot down balloons carrying propaganda leaflets critical of North Korea.

North Korea has bristled against anti-Pyongyang leaflets and South Korea's propaganda broadcasts near their tense border on fears that an influx of outside information could pose a threat to the Kim Jong-un regime.

North Korea has launched more than 3,000 trash-carrying balloons across the border over 10 occasions since late May in a tit-for-tat move against anti-Pyongyang leaflets sent by North Korean defectors and activists in the South.

Shin emphasized that trilateral security cooperation between South Korea, the United States and Japan against North Korea's threats has never been more important, and said the three nations are in the final stage of arranging a joint document that institutionalizes such efforts.

The envisioned document will encompass details on high-level exchanges as well as overall principles for their trilateral multidomain "Freedom Edge exercise" that was held for the first time in late June, according to Shin.

Shin is set to visit Tokyo for a trilateral meeting with his U.S. and Japanese counterparts slated for Sunday. (Yonhap)