A spokesman for the South Korean Defense Ministry tells Fox News shots fired by North Korean forces came from a North Korean  guardpost 1.3 kilometers (about 1300 yards, under a mile) inside the DMZ.

The North maintains positions in the “de-militarized” zone.

Officials tell us two rounds were fired using a heavy machine gun.  Three rounds were fired back from the South.    No casualties are reported on either side.

The exchange of fire happened at 5:26 pm local South Korean time Friday (4:26 a EDT).  A UN military team investigating the incident will be sent to the location tomorrow.

It happened about 30 miles east of the main Panmunjon DMZ crossing northeast of Seoul.

No casualties have been reported on either side.

Spokesman for US military in South Korea, David Oten, is downplaying the possible significance of the exchange of fire.

“I don’t want to call it ‘routine’” Oten said, “but this sort of thing happens about once a year.”

He went on to say, “…the shooting could have been accidental or a clearing of arms.”

As for the motivation for the exchange of fire, South Korean authorities are not committing.

“We are monitoring their behavior,” the Defense ministry spokesman told Fox , “and we are investigating the situation.”

A rejection by South Korea of new talks with the North might be a motivation, International Crisis Group’s Seoul-based representative Daniel Pinkston told Fox News.

The North has been concerned about loudspeakers and other propaganda moves by the South.   In the past it has threatened to attack the speakers with its artillery.

“They were very upset about those loudspeakers,” Pinkston told Fox.

South Korea rejected talks Thursday as it states the North has not yet apologized for its alleged sinking earlier this year of the SK ship Cheonan.

Friday, the North warned of “merciless physical retaliation” due to the South’s refusal.

The incident come just weeks after the formal “unveiling” of Kim Jong-un as the heir apparent to current North Korean leader Kim Jong-il.

“There is always the possibility that this is an effort to burnish the ‘Young General’s credentials,” Korea expert Peter Beck told Fox.

The incident also comes less than two weeks prior to the G-20 summit to be held in Seoul and to be attended by President Obama.

There have been unconfirmed reports the North was considering disrupting the talks and that Kim Jong-un  was involved in that planning.

South Korea is beefing up security in response to a possible threat.