South Korea is readying for another military drill against the North by mobilizing troops and other resources along the countries' heavily armed border. (Jo Yong-Hak/Reuters)

South Korea vowed Wednesday to "completely punish" North Korea if it attacks again, and mobilized hundreds of troops, tanks and helicopters for a massive military exercise prompted by high tensions on the peninsula.

The firing drills planned for Thursday near the Koreas' heavily armed land border signals South Korea's willingness to risk escalating tensions further with North Korea, which shelled a southern island off the western coast on Nov. 23.

The attack, which killed four people, was portrayed by Pyongyang as retaliation for southern military exercises on Yeonpyeong Island that day.

South Korea has conducted 47 similar military drills this year; it scheduled one more exercise for Thursday in response to the North Korean attack. Thursday's drill will be the biggest wintertime joint firing exercise South Korea's army and air force have staged, an army statement said.

"We will completely punish the enemy if it provokes us again like the shelling of Yeonpyeong Island," Brig. Gen. Ju Eun-sik, chief of the army's 1st armoured brigade, said separately.

North backs down on retaliation

South Korean forces are on high alert despite the North backing down from its threat to retaliate again over a separate firing drill the South held Monday on Yeonpyeong in disputed western waters.

The two Koreas remain technically at war since their conflict in the 1950s ended in a ceasefire, not a peace treaty. The past month's military tension, however, has been the worst in several years.

The North has made conciliatory remarks in recent days — telling a visiting U.S. governor it might allow international inspections of its nuclear programs — but Seoul is mindful of past surprise attacks and is still bracing for possible aggression.

South Korea's navy also began annual four-day firing and anti-submarine exercises Wednesday off the country's eastern coast. That area has been less tense recently but in the past, the North has used eastern waters as a submarine route for communist agents to infiltrate South Korea.

The Koreas' recent military skirmishes, including last month's artillery bombardment, have been in the tense western waters, where Pyongyang does not recognize the UN-drawn border.