South Korea's military has stepped up its combat readiness after detecting unusual military movement by North Korea's armed forces along the tense western sea border, Seoul officials said Wednesday.

Seoul recently discovered that the North's military had relocated a ground-to-air missile to north of the South's northernmost island of Baengnyeong. In addition, Pyongyang has moved the missile's movable launching pad to a missile base near the western sea border, a senior government official said, requesting anonymity. 

"We discovered that North Korea's army has recently moved its fighters from the rear to a base near the Northern Limit Line (NLL)," the official said, adding that several missiles were fired off the western and eastern coasts recently.

The official noted that the military movement is similar to the situation shortly before the North shelled the front-line island of Yeonpyeong in November last year, a surprise attack that killed two South Korean Marines and two South Korean civilians.

Yeonpyeong and Baengnyeong are located just south of the NLL, a de facto maritime border between the Koreas, and the waters around them have seen deadly naval skirmishes and armed provocations by North Korea.

In response to such activities, the Joint Chiefs of Staff has strengthened monitoring of the tense sea border and elevated the level of military preparedness of the Army, Navy and Air Force, officials said.

The move is in line with President Lee Myung-bak's order to maintain strong vigilance against the communist state while he is out of the country for talks with U.S. President Barack Obama along with Defense Minister Kim Kwan-jin. Lee left on Tuesday and is scheduled to return home on Sunday.

North Korea's latest military movement coincides with Pyongyang's threat of retaliation against propaganda leaflets sent by South Korea's conservative civic groups.

Just a day before the 66th anniversary of the ruling Workers'

Party, the North's state-run media, the Korean Central News Agency (KCNA), on Sunday reported that the North's army is "ready to destroy the citadels of psychological warfare."

On Saturday, the head of the North Korean delegation to the inter-Korean military talks sent a warning to Seoul, saying it would take physical action against the South's "ceaseless provocative war moves."

Two Koreas remain technically in a state of war because the

1950-53 Korean War ended in an armistice, rather than a peace treaty.