SEOUL, South Korea — 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 today near the Koreas’ heavily armed land border signaled that South Korea is willing to risk further escalating tensions with North Korea, which shelled a southern island off the western coast on Nov. 23 and stirred up a warlike atmosphere.

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

South Korea has conducted 47 similar military drills this year, and it scheduled one more exercise for today in response to the North Korean attack, an army officer said on condition of anonymity citing department rules. Today’s drill will be the biggest-ever wintertime joint firing exercise that 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 armored brigade, said separately.

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

The two Koreas remain technically at war since their 1950s conflict ended in a cease-fire, 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, but Seoul is mindful of past surprise attacks and still is 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.