South Korea will hit back with air strikes at the North and "punish the attacker thoroughly" should the regime launch another assault, the defence minister-designate warned Friday.

The tough words came as the largest ever US-Japan war games kicked off in waters off the tense Korean peninsula.

The manoeuvres in the East China Sea dwarf US-South Korean exercises this week in the Yellow Sea. These were designed as a show of force to Pyongyang after its regime launched a deadly artillery strike against South Korea.

In Seoul the nominated defence minister Kim Kwan-Jin told a parliamentary confirmation hearing that if the communist regime of Kim Jong-Il attacked again, "we would definitely use the air force to strike back".

The South's military counter-attacked with artillery after the North on November 23 shelled a border island, killing two civilians and two marines. But Seoul refrained from using air power for fear of escalating the clash.

The South's response was widely blasted as feeble and the previous defence minister announced he would step down to take responsibility.

Kim, a retired four-star general and former chairman of the Joint Chiefs of Staff, said the South would exercise its right to self-defence and "punish the attacker thoroughly until the source of hostility is eliminated".

Kim said the attack triggered "the most serious crisis" since the 1950-53 war. But he dismissed the chance of full-scale war as slim, citing the military prowess of South Korean and allied US forces, which have 28,500 troops based in the country.

The South also plans five days of artillery firing next week.

But the defence ministry held off on scheduling a drill on Yeonpyeong -- which was hit in the November 23 attack and is close to North Korea's coast -- saying it could come "by the end of this year at the latest".

Yonhap said the North had deployed 100 new multiple rocket launchers north of the border, boosting their number to 5,200. They have a range of 60 kilometres (37 miles), meaning they could hit the capital, Seoul.

The United States, meanwhile, joined forces with officially pacifist Japan in a giant display of military firepower dubbed "Keen Sword," with 60 warships, 500 aircraft and 44,000 troops in southern Japanese waters.

The long-scheduled drill comes in a year when China has had a bitter maritime territorial row with Japan and quarrelled with Southeast Asian nations over what the regional giant claims are its ancestral waters.

China -- which has resisted calls publicly to condemn its long-time ally North Korea for the artillery attack -- has instead called for negotiations with Pyongyang, saying that to talk is better than to "brandish weapons".

US Secretary of State Hillary Clinton -- who next week meets South Korea's and Japan's foreign ministers -- said she was discussing with Chinese and Russian officials "how we can work together to try to avoid conflict".

"North Korea poses an immediate threat to the region around it, particularly to South Korea and Japan, and a medium-term threat, should it collapse, to China," she told students on a visit to Kyrgyzstan.

She also said the North "poses a longer-term threat to the entire world because of its nuclear programme and its export of weapons around the world".

South Korea's President Lee Myung-Bak also mused about the downfall of the dictatorship.

"I think some positive changes are happening... North Koreans are allowed to keep vegetable gardens and private markets are popping up regardless of whether they are officially allowed or not," he said.

"Many defectors are coming (to the South). What we should pay attention to are changes among the North Korean people rather than changes among their leaders. No regime in history has been able to resist change among the people."

UN International Atomic Energy Agency chief Yukiya Amano in Vienna earlier expressed "great concern" over reports that the North -- which has twice tested atom bombs -- had built a modern new uranium enrichment plant.

A US scientist who toured the Yongbyon facility last month called it "stunning" and warned that, although it appeared to be for civilian use, it could be converted to make weapons-grade uranium.

Several South Korean anti-war activists rallied during a US army drill in Gwangyang port on the south coast, waving placards that read: "We don't want any military exercises. Let's realise peace on the Korean Peninsula!"

burs-fz/sm/bsk