South Korea's defence chief on Wednesday urged his soldiers to launch strong counter-attacks if provoked by the North as the two Koreas exchanged tirades despite a recent nuclear deal.

Defence Minister Kim Kwan-Jin said any fresh attacks from the North must be turned into a chance for the South's military to "avenge" the North's 2010 bombardment of a South Korean frontline island.

"If the North carries out any provocative act, you must punish them strongly by pounding not only the origins of such provocations but also their supporting units until they completely surrender," the minister said.

He made the remarks while visiting Yeonpyeong island near the disputed Yellow Sea border, against which the North mounted artillery attacks in 2010, killing four South Koreans and briefly sparking fears of war.

The shelling came amid high tensions following the North's alleged torpedoing of a South Korean warship, the Cheonan, in March 2009, which left 46 dead.

Kim's trip to Yeonpyeong came 10 days after North Korea's new leader Kim Jong-Un inspected a coastal artillery unit which shelled the island, urging soldiers to launch a "powerful retaliatory strike" if the South provokes.

"The military must retaliate strongly and thoroughly as a vengeance over the sinking of the Cheonan and the Yeonpyeong shelling," Kim said.

Pyongyang said last week it would suspend its nuclear tests and uranium enrichment programme in return for US food aid but ties with Seoul remain icy.

The communist state has threatened a "sacred war" against the South, labelling annual US-South Korea joint military drills a "silent declaration of war" and accusing the South of slandering and defaming its leaders.

"The North's recent hostile rhetoric... and more frequent visits to military units by its leadership indicate its power transfer has not been completed yet," the minister said.

South Korean officials said North Korea is likely to step up verbal attacks against the South in a bid to influence upcoming elections.

"The North is likely to ramp up tirades against the South in order to influence next month's parliamentary elections and the presidential poll (in December)," a presidential official told Yonhap news agency.

Professor Kim Yong-Hyun of Seoul's Dongguk University said the North was unlikely to translate verbal attacks into military action for fear that it would derail diplomatic efforts.

Cheong Seong-Chang of the Sejong Institute said, however, Jong-Un was "much more adventurist" than his late father, urging the South to stay on its guard against possible military attacks.

Pyongyang's resentment reached a boiling point this month after a military unit west of Seoul allegedly hung up portraits of the North's leader and his father with slogans such as "Beat Kim Jong-Un to death".

Massive rallies have been held across the North, with participants pouring invective against the South's President Lee Myung-bak, calling him a "mad dog" or a "psycho".

On Tuesday, the North's state television aired footage of North Korean soldiers firing at Lee's name written on a target and hurling daggers at another target inscribed with the name of the South's defence minister.