The threatened nation carried out massive live-fire drills to demonstrate its readiness to retaliate against North Korean aggression.
It comes as South Korean President Moon Jae-in said “now is not the time” for peace talks with the hermit dictatorship.
The wargames commenced as it emerged South Korea was considering moving US nuclear weapons back on to its territory for the first time since the early 1990s.
Moon advocated dialogue with North Korea during his run for President earlier this year, but has reversed his position in the wake of provocative nuclear testing by Pyongyang.
He was under pressure to take a harder line as there had been no change in the rapid pace of North Korea’s ballistic missile testing since he took office.
Moon emphasised that he remains interested in dialogue with North Korea at a later date.
The two nations are still technically at war as the Korean War of 1950-53 ended in a truce, rather than a peace treaty.
Both sides have huge military arsenals aimed at one another but North Korea’s six recent nuclear tests left the two nations, and the world, on the brink.
As well as huge navy drills, South Korea and the US have now agreed to scrap a weight limit on its warheads.
South Korea is also considering moving aircraft carriers and strategic bombers close to the border between the two nations.
Meanwhile Vladimir Putin has said the stand off between North Korea and the US could spark a “global catastrophe”.
The Russian President urged Washington and Pyongyang to tone down their rhetoric and warned the UN not to introduce “useless” additional sanctions against the hermit regime.
German Chancellor Angela Merkel and US President Donald Trump have urged additional sanctions, but Putin said sanctions would not stop Kim Jong-un from threatening western nations.
He said: “The North Korean Government is not going to change their policy, whereas millions of people will suffer.”
President Donald Trump recently threatened to unleash “fire and fury” if Kim continued to threaten to attack the US island of Guam.
Mr Putin said: “Ramping up military hysteria in such conditions is senseless, it’s a dead end. It could lead to a global catastrophe and a huge loss of human life.
“There is no other way to solve the North Korean nuclear issue, save that of peaceful dialogue.”