The Air Force KF-16 jets were participating in joint US-South Korea live-fire training exercises in Pocheon, about 25 kilometres (16 miles) south of the heavily fortified border with nuclear-armed North Korea, military and government officials said.

The two jets accidentally released eight MK-82 general-purpose bombs -- four each -- which landed "outside the designated firing range", the Air Force said.

"The pilot appears to have entered incorrect coordinates during the pre-flight preparation process," an Air Force official told reporters.

One Pocheon resident told Yonhap news agency he was watching television when he heard the bombs explode.

"I suddenly heard an enormous explosion, like a thunderclap, and the whole house shook," said the resident, who gave only his surname Park.

"When I went outside, everything was in chaos," he said.

The two aircraft were meant to fire simultaneously at a target as part of the exercise, the Air Force official said.

"It is believed that the first pilot entered incorrect coordinates, leading the second aircraft to drop its munitions as well."

A defence ministry official said an investigation was underway.

"We deeply regret the unintended release of the bombs, which resulted in civilian casualties, and wish those injured a swift recovery," the Air Force said in an earlier statement.

It said it would "take all necessary measures, including compensation for damages".

South Korea's National Fire Agency said the bombs had "fallen on a village during a South Korea-US joint exercise".

That resulted in "casualties and property damage, with many displaced residents", it said.

Fifteen people were injured, including civilians and military personnel, the agency said.

One church building and sections of two houses were damaged, according to an agency statement.

- 'Simply unimaginable' -

The mayor of Pocheon demanded that all military exercises cease until preventive measures were made.

"While there have been accidents where bullets bounced off targets and landed elsewhere, it is simply unimaginable that an aircraft would directly fire at civilian homes," said mayor Baek Young-hyeun.

Baek told reporters that "until a thorough and acceptable safety plan is established to the satisfaction of Pocheon residents, all military exercises must be suspended".

"I strongly urge the government to recognise the gravity of this incident and act swiftly in response," Baek said.

The blast was felt at a centre for seniors about a kilometre away.

"A sudden explosion shook the building. The windows shattered, and one of our teachers was injured and taken to the hospital," the centre's director, surnamed Yu, told Yonhap.

"Fortunately, none of the seniors were hurt but they were so frightened that we sent them all home," Yu said.

The South Korea-US "Freedom Shield" military exercises, one of the allies' largest annual joint exercises, are set to begin later this month.

The two Koreas remain technically at war since their 1950-1953 conflict ended in an armistice, not a peace treaty.

The United States stations tens of thousands of soldiers in the South, in part to protect Seoul against Pyongyang.

In 2022, a South Korean Hyunmu-2 short-range ballistic missile accidentally crashed into a military golf course in the South's eastern Gangwon province, after it was fired in response to a North Korean missile launch.

The missile did not explode and no one was injured in that incident.