South Korea's top military officer called for a watertight combat readiness Thursday against North Korea's provocations, vowing to retaliate against them without hesitation, military officials said.

Adm. Choi Yun-hee, chairman of the Joint Chiefs of Staff, made the remarks during his visit to a Marine Corps unit on Yeonpyeong Island in the Yellow Sea, which North Korea shelled in November 2010, killing two soldiers and two civilians.

"There is the need to thoroughly brace for any blitz provocations by North Korea," Choi said. "If the North makes provocations, we should retaliate against the North without hesitation and mercy."

The remarks came as North Korea has ratcheted up its provocations in recent weeks by test-firing what it claimed to be a submarine-launched ballistic missile. It also said that it has already miniaturized nuclear warheads enough to be placed on missiles.

"There is a high possibility that the North's military would make a surprise provocation in a different style under its carefully drawn plans," Choi said. "We need to maintain watertight combat readiness against the North's threats."

He also ordered the Marine Corps to protect South Korean fishing vessels to prevent them from being seized or attacked by North Korea. (Yonhap)