North Korea denounced the exercises as dangerous "when insane President Trump is running wild," while analysts warned that they sharply increase the chances of miscalculation and accidental conflict.
More than 230 warplanes - including six U.S. Air Force F-22 Raptors and another six F-35A stealth fighters deployed to the peninsula especially for the exercises - are taking part.
Two B-1B Lancer bombers are also expected to participate in the five-day-long drills, called "Vigilant ACE," along with about 12,000 American troops, including Navy and Marine Corps personnel.
The "realistic air combat exercise" is designed to enhance interoperability between the U.S. and South Korea and increase the allies' combat effectiveness, the U.S. Seventh Air Force said in a statement.
They will practice for a range of wartime scenarios, including enemy infiltration drills and precision strikes on mock North Korean nuclear and missile targets, the statement added.
The drills will take place in all weather, in both day and night, so the forces are ready for any situation, South Korea's Defense Ministry said.
However, the drills got off to an inauspicious start when one of the Raptors touched down at a South Korean base in Gwangju on Monday morning but then couldn't move and had to be towed, a South Korean official told Stars and Stripes. The Yonhap News Agency published photos of an F-22 being towed.
The third annual drills were scheduled long before the recent increase in tensions between North Korea and the outside world, and they are comparable in size to previous years, although fifth-generation stealth jets are involved for the first time this year.
Still, the environment could hardly be more tense.
After detonating a hydrogen bomb in September, North Korea last week launched its most advanced intercontinental ballistic missile yet, one that puts all of the United States within range. Kim Jong Un's regime proudly announced that it had now completed its nuclear goals.
This prompted an angry reaction from Washington and a threat of further sanctions.
But it has also led to another spate of warnings that, if sanctions don't change North Korea's calculus, military action might be needed.
H.R. McMaster, President Trump's national security adviser, said over the weekend that the possibility of war with North Korea was increasing every day that the nuclear crisis wasn't solved.
"The priorities that the president's given us to move as quickly as we can to resolve this crisis with North Korea," McMaster told Fox News in an interview Sunday.
"If necessary, the president and the United States will have to take care of it, because he has said he's not going to allow this murderous, rogue regime to threaten the United States," he said.
Separately, Republican Sen. Lindsey O. Graham, a member of the Senate Armed Services Committee, said U.S. military families in South Korea are to leave because a pre-emptive attack on North Korea is "becoming more likely."
"We're getting close to a military conflict because North Korea's marching toward marrying up the technology of an ICBM with a nuclear weapon on top that can not only get to America but deliver the weapon. We're running out of time," Graham said on CBS News' "Face the Nation" Sunday.
Before these comments and before the exercises began, North Korea said the drills showed that the U.S. and South Korea were "aggressors and provocateurs" and that Pyongyang was right to develop its "deterrent" nuclear program.
"The projected war rehearsal is just a grave military provocation which will push the already acute situation on the Korean Peninsula to the brink of nuclear war," the North's Committee for the Peaceful Reunification of the Country, which handled relations with South Korea, said in a statement carried by the official news agency.
The provocative exercises "will only invite more terrible retaliation and precipitate their self-destruction," the committee warned the U.S. and South Korea.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)