North Korea's leader Kim Jong-un, center, visits military artillery school in North Korea, in this photo provided by North Korea's official Korean Central News Agency, Oct. 6 Yonhap

North Korea is likely to deploy members of its regular armed forces to Ukraine in support of Russia, South Korea's defense chief said Tuesday, in the latest sign of deepening military cooperation between Pyongyang and Moscow.

The assessment came as North Korea has been seeking to bolster ties with Russia, highlighted by a bilateral agreement that includes a mutual defense clause, signed by North Korean leader Kim Jong-un and Russian President Vladimir Putin at summit talks in June.

"As Russia and North Korea have signed a mutual treaty akin to a military alliance, the possibility of such a deployment is highly likely," Defense Minister Kim Yong-hyun told lawmakers during a parliamentary audit session.

Against this backdrop, Kim said a recent report on North Korean troop casualties in a Russian-occupied territory near the eastern Ukrainian city of Donetsk is also likely to be true.

Last week, the Kyiv Post reported that six North Korean officers were killed and three others were wounded in a Ukrainian missile strike on Oct. 3, citing intelligence sources.

When asked about the possibility of North Korea declaring a new maritime border further south of the current de facto sea border, known as the Northern Limit Line (NLL), Kim vowed a firm response in such an event.

"Regardless of what line they claim, the current NLL is a line that we have guarded with blood. We will not allow any other line than this at all and sternly retaliate," Kim said.

His remarks came as North Korea appears to have convened a key parliamentary meeting Monday to revise its constitution as the North's leader has called for removing unification-related clauses and clarifying the country's territorial boundaries.

Experts said North Korea could unilaterally announce a new maritime border south of the NLL in a bid to use it as an excuse to stage provocations. (Yonhap)