While months-long winter drills are routine for the North, authorities in the South said they were on high alert for any possible changes.
"Currently, the North Korean troops are conducting ordinary field exercises", a South Korean military official said in a briefing, according to Yonhap.
"That might be a unusual thing to happen but it's a possibility", he said.
His letter comes after the Pentagon claimed North Korea's nuclear arsenal could only be eliminated with a ground invasion.
USA president Donald Trump has insisted that by calling him old, North Korean leader Kim Jong-un had insulted him, adding that he would never stoop to the same level by calling Kim "short and fat".
Lieutenant General Jan-Marc Jouas, the former Deputy Commander of United States forces in Korea, said the USA would not even be able to build-up their forces before a war took place, in a chilling letter to the Democrats, seen by Newsweek. We will defend our common security, our shared prosperity and our sacred liberty.
The three flattops are the USS Ronald Reagan, Nimitz and Theodore Roosevelt.
Mr Trump spoke on North Korea in Vietnam on Sunday, saying "it would be very, very nice" if he and Kim became friends.
"The large-scale nuclear war exercises and blackmails, which the USA staged for a whole year without a break in collaboration with its followers to stifle our republic, make one conclude that the option we have taken was the right one and we should go along the way to the last", Ja said.
President Trump is now in the Philippines for the ASEAN Summit, the last leg of his Asia tour.
In an effort to rally global pressure against North Korea's nuclear weapons program, Trump had said during his speech in South Korea on Tuesday, "Do not underestimate us".
North Korea complained to the United Nations on Monday about joint military exercises by the United State and South Korea, describing it as "the worst ever situation" because U.S. nuclear war equipment had been deployed ready to strike.
South Korean soldiers ride on an armored vehicle during a South Korea/United States combined training exercise at the U.S. Army's Rodriguez shooting range in Pocheon, northeast of Seoul, Sep.