South Korean and US soldiers have taken part in the largest military exercises between the two countries for more than 20 years.
The 12-day amphibious landing drill that began on Thursday features 15,000 South Korean soldiers and 10,000 US troops, including marines. The operation, code-named Ssan Yong or Twin Dragons, comes a day after North Korea launched two mid-range ballistic missiles.
The exercises on South Korea's southeastern coast are the biggest since the annual Team Spirit exercises held between 1976 and 1993.
There are nearly 30,000 US troops permanently stationed in South Korea and the two countries conduct annual military drills.
North Korea view such drills as provocative rehearsals for invasion, heightening the risk of further military tensions between the North and its neighbours.
Pyongyang has carried out a series of rocket and short-range missile launches in recent weeks, sparking condemnation from Seoul and Washington.
United Nations resolutions prohibit North Korea from conducting ballistic missile tests and the UN Security Council will hold closed-door consultations on Thursday to discuss a possible condemnation of the latest missile launches,
Earlier this month, the North's powerful National Defence Commission threatened to "demonstrate" its nuclear deterrent in the face of what it called US hostility.
South Korea's Defence Ministry warned that North Korea could be working towards a major provocation but added there were "no signs" of an imminent nuclear test.
Pyongyang staged three nuclear tests in 2006, 2009 and 2013.