South Korea completed a major show of its military strength amid high tensions with rival neighbor North Korea.
North Korea has said it is ready to wage a "holy war" against the South using its nuclear deterrent.
The threat, by North Korea's minister of armed forces, Kim Yong-chun, came just hours after South Korea completed military land exercises near their shared border.
South Korean officials had said that country's live-fire land drill in the Pocheon region, between the South Korean capital and the heavily armed Demilitarized Zone separating the two Koreas, would involve hundreds of troops as well as tanks, helicopters, and fighter jets.
After the drill, South Korea's President Lee Myung-bak threatened to launch a "merciless counterattack" against North Korea if it again strikes targets in the south. Lee also warned that it is a mistake to think that patience will bring about peace with North Korea.
North Korea criticized the land drill and naval exercises that began on December 22, but had stopped short of threatening retaliatory strikes. North Korea did not carry out its threat to retaliate against the naval exercises earlier this week.
The South's exercises -- described as some of the biggest ever held by South Korea -- come exactly one month after North Korea shelled South Korea's Yeonpyeong Island, killing four South Koreans, in an escalation of hostilities between the rivals.
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