South Korea's military said on Tuesday that it had fired warning shots at the weekend after some North Korean soldiers briefly crossed the border.

"Some North Korean soldiers working within the DMZ on the central front briefly crossed the Military Demarcation Line," the South Korean joint chiefs of staff (JCS) said in a statement.

It was referring to the line of control in the heavily fortified border between the two Koreas.

"After our military issued warning broadcasts and warning shots, they retreated northward," it said, adding the incident happened June 9.

"Apart from the immediate retreat of the North Korean soldiers following our warning shots, there were no unusual movements observed," the JCS said, adding the military was closely monitoring troops near the border.

What's behind tensions between the two Koreas?

The incident came amid heightened tensions on the Korean Peninsula over Pyongyang's launch of balloons carrying trash like cigarette butts and toilet paper into the South.

The North said the balloons were a response to a campaign by North Korean defectors in the South who have flown balloons carrying anti-Pyongyang leaflets in the other direction over the border.

In response to the trash balloons, the South Korean government this month also suspended a 2018 tension-reducing military deal and restarted loudspeaker propaganda broadcasts along the border.

The move irked Pyongyang, which warned Seoul was creating "a new crisis."

North installing loud speakers?

South Korea's military said Monday it had detected signs the North was installing its own loudspeakers.

North Korea had used loudspeakers along the border since the 1960s, typically broadcasting praise of the Kim family.

But Pyongyang suspended their use in 2018 as ties between the two Koreas warmed at the time.

But relations between the two sides have deteriorated over the past couple of years.

The two Koreas remain technically at war since the 1950-53 conflict, which ended in an armistice rather than a peace treaty.