South Korea on Wednesday prepared for a major show of military strength involving fighter jets and tanks near the tense North Korean border as Washington ruled out "feel-good" talks with Pyongyang.

The live-fire exercise, planned for Thursday with self-propelled guns and 800 soldiers, follows signs of an easing of tensions on the peninsula after the North backed down from a threat to retaliate against an earlier drill.

Although similar exercises have been held at the same firing range 20 kilometres (12 miles) south of the mainland border many times before, the latest comes with Seoul on high alert for a possible attack from the North.

South Korea's navy meanwhile began a four-day firing drill Wednesday off the east coast, a relatively distant 100 kilometres (60 miles) south of the border with the North, mobilising six warships plus helicopters.

Observers said South Korea's conservative government sees flexing its military muscles as a necessary deterrent against a fresh strike by the North.

"If you relax and are unprepared and try to be nice to the North Koreans they will never stop," said Daniel Pinkston, an expert at the International Crisis Group think-tank in Seoul. "It's like a bully in the schoolyard."

A senior South Korean military commander said Thursday's drill at the Pocheon range would "demonstrate our solid military preparedness".

"We will retaliate thoroughly if the North commits another provocative act," First Armoured Battalion commander Choo Eun-Sik told Yonhap news agency.