POCHEON, South Korea — South Korean fighter jets dropped bombs and tanks fired artillery Thursday as the military staged its largest air-and-ground firing drills of the year in a show of force a month after North Korea's deadly shelling of a front-line island.The drills, at training grounds in mountainous Pocheon about 20 miles (30 kilometers) from the Koreas' heavily fortified border, signaled South Korea's determination to demonstrate and hone its military strength at the risk of further escalation with North Korea.Tanks raced down mountain roads firing artillery rounds. The boom of cannons echoed through the valley and the hills erupted in smoke. Rockets streamed across the valley and slammed into the side of a hill as helicopters overhead fired rockets at targets and F-15 fighters zoomed by dropping bombs.The drills, which lasted less than 45 minutes, were the armed forces' largest joint firing exercises this year, and the biggest-ever wintertime air and ground firing exercises in terms of the number of weapons mobilized and fired, government and army officials said on condition of anonymity, citing department rules.Forty-seven similar exercises have taken place this year but Thursday's maneuvers were scheduled in response to the North Korean attack, according to army officials.Exactly one month ago, routine South Korean live-fire drills from Yeonpyeong Island in the Yellow Sea triggered a shower of North Korean artillery that killed two marines and two construction workers. It was the first military attack on a civilian area since the 1950-53 Korean War ended in a truce.North Korea, which claims the waters around the South Korean-held island lying just 7 miles (11 kilometers) from its shores as its territory, accused the South of sparking the exchange by ignoring Pyongyang's warnings against staging the live-fire drills near their disputed maritime border.Amid international concerns of all-out war on the tense Korean peninsula, South Korea has pushed ahead with military exercises over the past several weeks, including live-fire drills from Yeonpyeong Island and Thursday's exercises."We will thoroughly punish the enemy if it provokes us again as with the shelling of Yeonpyeong Island," Brig. Gen. Ju Eun-sik, chief of the South Korean army's 1st Armored Brigade, said in a statement Wednesday.North Korea issued a statement calling the South Korean drills "provocative" and "offensive," state-run media said. However, the statement, carried by the official Korean Central News Agency, did not threaten retaliation.The two Koreas remain technically at war because their 1950s conflict ended in a cease-fire, not a peace treaty.The military tension over the past month has been the worst in more than a decade, and comes on the heels of the March sinking of a South Korean warship that Seoul blames on Pyongyang, but which North Korea denies attacking. Forty-six sailors died in that incident.South Korea's navy also was conducting annual anti-submarine exercises off the east coast.In Pocheon, about 800 local residents, including schoolchildren in bright yellow jackets, were invited to watch the drills."We are facing a crisis because of North Korea, so I came to see this air and ground operation. I want to feel and see the level of South Korea's armed forces," said Kim Tae-dong, 70. "Another North Korean provocation will happen. We should prepare our military perfectly for that."Separately, President Lee Myung-bak visited a front-line army base near the Koreas' eastern land border to review military readiness and boost troop morale, his office said.