Dozens of South Korean warships, fighter jets and submarines are taking part in live-fire drills, in what Seoul says is a response to recent military provocations by the North.
South Korea's defense ministry said the joint navy and air force drills involved at least 20 vessels and included the firing of anti-ship missiles off the east coast of the Korean peninsula.
The Tuesday exercises are "aimed at quashing the North's intention for provocation at sea," according to defense ministry spokesman Na Seung-Yong.
Seoul officials have expressed concern after North Korea said this month it test-fired a submarine-launched ballistic missile, or SLBM, off its west coast.
The reports of the test could not be confirmed and experts have stressed they believe Pyongyang is still years away from fully developing the technology.
When completed, a submarine-based missile program could significantly expand the reach of North Korea's nuclear weapons.
North Korea is banned under U.N. sanctions from conducting ballistic missile or nuclear tests, though it has pushed ahead with developing the weapons programs in recent years.
U.N. Secretary-General Ban Ki-moon on Monday called on North Korea to "take steps necessary to prevent escalation," warning that if Pyongyang's current activities continue, the region could see "more arms competition and rising tension."
Speaking at a conference in Seoul, Ban also called on North Korea to "enable a return to multilateral negotiations and engagement, including by complying with all relevant Security Council resolutions."
In 2009, North Korea walked out of six-nation talks aimed at getting Pyongyang to abandon its nuclear program in exchange for economic aid. It has since expressed a desire to return to the dialogue, but has refused Washington's condition that it must first commit to giving up its nuclear program.
Pyongyang's government, which is still technically at war with the South, has spent hundreds of millions of dollars on the nuclear and missile programs, which it sees as essential to its survival.