South Korea has said it will hold new large-scale military drills involving ground and air live fire on Thursday.
Artillery, jets and about 800 soldiers will take part, the government said, alongside separate naval exercises that began on Wednesday.
Tension has been high since North Korea shelled the South's Yeonpyeong island last month, killing four South Koreans.
Drills by the South on Monday near Yeonpyeong sparked Northern threats of retaliation that did not materialise.
An army spokesman said Thursday's drill would be held at Pocheon, 20km (12 miles) south of the border - about 50km from central Seoul.
Exercises have been held at Pocheon before, but this would be on an unprecedented scale, the spokesman said.
"The scale of mechanised assets taking place is enormous. When we would normally have 6 K-9 mechanised artillery, we'll have 36.
"We'll have the F-15 jets firing. We'll have choppers. You can say most of the mechanised assets taking part will be firing live ammunition," the spokesman said.
"We will retaliate thoroughly if the North commits another provocative act like the shelling of Yeonpyeong," First Armoured Battalion commander Choo Eun-sik told Yonhap news agency.
"Through this exercise [at Pocheon], we will demonstrate our solid military preparedness," he said.
The BBC's Kevin Kim in Seoul says this is the largest winter live-fire exercise ever conducted on land here.
Separately, a "routine" four-day naval firing exercise has begun off the east coast of South Korea, involving six warships and helicopters.
The North Korean shelling of Yeonpyeong shocked South Koreans.
It sparked the replacement of the country's defence minister and the development of a more active defence and deterrence policy among South Korean planners.
South Korea and the US - with which it has a long military relationship - 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.
The pace of military drills has been stepped up in recent weeks, despite frequent denunciations from North Korea and its closest ally China.
Meanwhile on Tuesday, a giant Christmas tree was lit on the southern side of the demilitarized zone, despite concerns in Seoul that North Korea could strike the position.
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.
After a visit to North Korea, the US politician, New Mexico Governor Bill Richardson, said North Korea agreed to let international monitors back into the country to inspect its nuclear sites.
China has also urged the North to invite staff from the International Atomic Energy Agency but there has been no word from the North on the subject.
"The six-party talks will be restarted again when the North Koreans display a willingness to change their behaviour," White House spokesman Robert Gibbs said.
North Korea walked out of the six-party talks in April 2009 and expelled UN nuclear inspectors from the country.