South Korea is keeping closer tabs on the possibility of North Korean provocations as its military is seen preparing for a large-scale exercise on its east coast and increasing frontline defense and submarine activities on its west.

The rise in North Korean military activities came as the South and the U.S. have begun their annual war drills, which Pyongyang has condemned as a “rehearsal for an incursion to topple its regime.”

The allied Foal Eagle field training exercise, which began on Friday, will continue through April 30 while the Key Resolve command post exercise will run from March 11 to 21. It is rare for the North to time its exercise to coincide with the annual drills.

In particular, Seoul is paying closer attention to the North’s submarine activities. Military sources said they had found its midget submarines began fresh operations near the Northern Limit Line, a de facto inter-Korean sea border.

Some experts dismissed the concerns over the submarine activities as part of usual movements that come during the spring thaw.

The military authorities are trying to track the movements of the North’s Taedong-2 class submarines equipped with two torpedoes each. In March 2010, a North Korean midget submarine torpedoed the South Korean corvette Cheonan, killing 46 sailors.

During his visit to the Navy’s Second Fleet on Saturday, Seoul’s Joint Chiefs of Staff chairman Jung Seung-jo directed his troops to prepare against the threat posed by the submarines.

“The enemy could definitely launch (additional) provocations,” said Jung.

“We should stay alert and recognize that the North could launch the provocations in our northwestern waters. We should step up our monitoring and patrols so that we would not allow for any loopholes (for the enemy) to launch any surprise attacks.”

The North Korean military is also seen readying its coastal artillery pieces heading toward the South for unknown reasons, and frequently conducting maintenance-related activities in its artillery units.

Regarding the North’s preparations for its upcoming exercise that involves all armed services, observers believe it would be staged in a show of force against the allied drills in the South and at the same time, to present their anger over the recent movements in the international community to sanction the North for its third nuclear test on Feb. 12.

Defense Ministry spokesperson Kim Min-seok said Pyongyang appeared to have raised the scale of its military exercise after its long-range rocket launch in December and its third nuclear test last month.

“We could say that the North prepares for a large exercise in response to the allied drills. But given various circumstances, it is judged that the North has expanded its conventional military drills following the missile and nuclear tests,” he said during a regular press briefing.

Seoul is also following up on the North’s activities around its northeast Punggye-ri nuclear test site. There have reportedly continued to be “some activities” in the southern part of the underground test site. Pyongyang has threatened to launch additional nuclear tests.

By Song Sang-ho (sshluck@heraldcorp.com)