His letter comes after the Pentagon claimed North Korea's nuclear arsenal could only be eliminated with a ground invasion.
Seoul's military said in a statement that the exercises aim to enhance the combined US and South Korean operational and aerial strike capabilities and to display "strong will and firm military readiness to defeat any provocation by North Korea with dominant force in the event of crisis".
News about the North's manoeuvres has emerged at a time when three of the US Navy's aircraft carriers - USS Ronald Reagan, USS Theodore Roosevelt and USS Nimitz - are holding maritime exercises in the region.
"Multiple carrier strike force operations are very complex, and this exercise in the Western Pacific is a strong testament to the US Pacific Fleet's unique ability and ironclad commitment to the continued security and stability of the region", Adm. Scott Swift, the commander of the US Pacific Fleet, said in a statement.
Along with the three carrier groups, he said the US has reactivated round-the-clock sorties with nuclear-capable B-52 strategic bombers "which existed during the Cold War times".
Tensions between North Korea and the U.S. have escalated in recent months following the rogue state's sixth nuclear bomb test on September 3, which earned more sanctions and condemnations from the UN. The weapons you're acquiring are not making you safer, they are putting your regime in grave danger.
Trump asked Xi Jinping to help UCLA basketball players busted shoplifting
White House press secretary Sarah Huckabee Sanders confirmed in an email to the newspaper that Trump raised the issue with Xi. The source also said there is surveillance video that shows the players allegedly shoplifting from three stores.
Arab League To Meet On Iran at Saudi Request
Iran says its missile program is defensive and it has no plans to build nuclear-capable missiles. But Iranian Foreign Ministry spokesman Bahram Qassemi rejected that possibility.
Butch Jones out as Tennessee's football coach
He can certainly provide a steadying hand, drawing on previous head coaching experience at Ball State, San Diego State and MI . He'll be owed a buyout in excess of $8 million, although he's required to seek comparable employment to mitigate the damages.
Trump said in a speech at South Korea's parliament Wednesday, "Currently stationed in the vicinity of this peninsula are the three largest aircraft carriers in the world loaded to the maximum with magnificent F-35 and F-18 fighter jets".
But Pyongyang doesn't seem interested in negotating.
He also said Chinese President Xi Jinping was "upping sanctions" against North Korea in response to its nuclear and missile programs. The statement also referred to Trump as a "dotard", an insulting term for old people who are going senile.
Asked if he could see himself being friends with Kim, Trump said: "That might be a odd thing to happen but it's a possibility".