South Korea has staged one of its largest-ever military drills amid highly tense relations with the North.
Tanks, helicopters, fighter planes and hundreds of troops took part in the excercise, about 20km (12 miles) from the heavily militarised land border.
Tension has been high since North Korea shelled the South's Yeonpyeong island last month, killing four South Koreans.
South Korean President Lee Myung-bak promised immediate retalation to any further Northern attack.
"I thought patience would bring peace to this land, but [the reality] was not so," he said in a statement, after visiting a military unit near the border to inspect defences.
"We can prevent [the North] from taking provocative acts through strong unity coupled with powerful response."
'Drills for invasion'
The North has branded Seoul's exercises "warmongering" but has not threatened the South with any retaliation.
The South Korean army acknowledged that the drill was aimed to display its full firepower.
"Yes, it will be a show of force," one army officer told Associated Press news agency ahead of the exercise.
Thursday's drill was held at Pocheon, about 50km from central Seoul.
Exercises have been held at Pocheon before, but this was on an unprecedented scale, an army spokesman said.
Although the South has conducted 47 military drills this year, this is the largest winter live-fire exercise ever conducted on land.
It began about 45 minutes later than the scheduled start time of 1400 local time (0500 GMT) and lasted about 40 minutes, with 800 troops and more than 100 types of weapons deployed, including tanks, anti-tank missiles, helicopters and fighter jets.
Artillery rounds and rockets were fired, bombs dropped and hillsides erupted in smoke.
The BBC's Kevin Kim in Seoul says the exercise sounded dramatic but was more of a media event, showing the range of weaponry open to the South, with invited members of the public watching from a distance.
He says people in Seoul are carrying on as normal with tension reduced from earlier in the week.
When the North fired on Yeonpyeong last month it said it was in response to a live-fire drill by the South.
A statement by the North's official KCNA news agency said of the latest drills: "[South Korea] is trying to hide the provocative nature toward the North of the war exercises."
The South is simultaneously conducting four days of naval exercises. The North called those "fanatical drills for invasion".
But the comments have been far less strident than threats issued last week of retaliation over the South's impending military drills.
White House spokesman Robert Gibbs said: "Exercises that have been announced well in advance, that are transparent, that are defensive in nature, should in no way engender a response from the North Koreans."
The BBC's Charles Scanlon says North Korea's leaders may now conclude that their artillery attack last month has served its purpose - South Korean generals are now matching their Northern rivals with hostile rhetoric, a dramatic change of mood in the South.
The South's new, more aggressive stance, our correspondent adds, has given North Korea an opportunity to present itself as the injured party and a force for restraint.
Already, both China and Russia have called on the South to defuse tensions and US officials too are privately expressing their concern.
South Korea and the US had already been conducting large-scale military exercises, following the apparent torpedoing of a South Korean warship by the North on 26 March, which killed 46 south Korean sailors.
Efforts to redirect the Korean issue back to the negotiating table have been unsuccessful.
China and the North say it is time to return to the six-nation talks about North Korea's nuclear programmes.
But the US, South Korea and Japan have said they will not return to such talks, which have previously involved rewards for the North if it cuts back on nuclear development.
North Korea walked out of the six-party talks in April 2009 and expelled UN nuclear inspectors from the country.