Several North Korean submarines have disappeared after departing from bases on the eastern coast.

According to reports, three or four North Korean submarines recently departed from bases on the east coast and have since remained outside South Korean surveillance.

The submarines are thought to be 370-ton shark-class vessels. The North Korean submarine used in the attack on the Cheonan in March 2010 was a 130-ton salmon-class vessel. North Korea has between 70 and 80 submarines ranging from 1,500-ton vessels to 130-ton vessels. Of the total, 80 percent are said to be based in the East Sea, where the waters are deeper than the West Sea, providing better conditions for submarine operations.

“North Korea appears to be increasing submarine infiltration exercises with the weather getting warmer. However, the possibility that drills could hide provocations has not been ruled out and related activities are being closely monitored,” an unnamed military source was quoted as saying by a local daily. The Navy’s public relations department did not confirm the reports.

In a separate development, North Korean leader Kim Jong-un visited an outpost on an East Sea island, the North Korean state media Korean Central News Agency reported on Wednesday.

The news agency reported that Kim told the troops stationed on Ryo Islet to “bury all enemies at sea” if a confrontation occurs, and to strengthen the defenses of the islet as “an iron wall” to prevent invasions.

The visit to the East Sea base was Kim’s first public visit to a military base since he attended an army-navy-air force joint exercise on March 15.

The submarine movement and Kim’s visit come as tensions have risen on the Korean peninsula due to Pyongyang’s plans to launch a long-range rocket supposedly carrying a satellite in April. The plans have been condemned by South Korea and the U.S. Pyongyang has responded by threatening a third nuclear test should the U.S. and its allies interfere with its plans.

By Choi He-suk (cheesuk@heraldcorp.com )