SEOUL, South Korea — North Korea fired several ballistic missiles into the sea on Monday, South Korea’s military said, hours after South Korean and U.S. troops kicked off their large annual combined drills, which the North views as an invasion rehearsal.

At least 15 people were injured after South Korean fighter jets involved in a live-fire drill dropped bombs on Pocheon, Seoul's military said.

South Korean fighter jets mistakenly dropped eight bombs on a civilian district on Thursday, injuring 15 people and damaging houses and a church during military exercises in Pocheon, the Air Force and fire agency said.

The incident took place at 10:04 am (local time), and the Air Force assured that it was investigating the accident. The authorities also apologised for the damage caused to the civilians

North Korea on Monday condemned joint U.S.-South Korean military drills as a "provocative act," warning of the danger of sparking war with "an accidental single shot," days after Seoul's air force mistakenly bombed a village on its own territory.

The North Korea Foreign Ministry said U.S.-South Korea joint military exercises are "a dangerous provocative act," state media KCNA reported on Monday. Annual South Korean and U.S. military exercises called Freedom Shield are due to begin on Monday and run until March 20.