The drills always raise tensions on the divided Korean peninsula and this year they come after the North's recent ballistic missile test and the assassination by nerve agent of Kim's half brother in Malaysia.
Participation in the joint exercises -- known as Key Resolve and Foal Eagle -- is similar to last year, a United States Forces Korea spokesman said.
The 2016 drills involved 300,000 South Korean and around 17,000 US troops, as well as strategic US naval vessels and air force assets.
South Korean Defence Minister Han Min-Koo "stressed the need to bolster the drills" in a phone call with his US counterpart James Mattis, who vowed "effective and overwhelming response" against any use of nuclear weapons, Seoul's defence ministry said in a statement.
Dozens of protestors gathered outside the US embassy in Seoul today protesting the start of the exercises, claiming they would "bring the peninsula sharply closer to the brink of a nuclear war".
Visiting the headquarters of an army unit, the North's leader Kim praised his troops for their "vigilance against the US and South Korean enemy forces that are making frantic efforts for invasion", the state-run Korean Central News Agency said today.
Kim also ordered the troops to "set up thorough countermeasures of a merciless strike against the enemy's sudden air assault", it said.
South Korea's acting president said Wednesday the country would respond with strong actions against any provocations made by the North and seek tougher UN sanctions against the isolated state.
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