Newly-planted landmines by North Korea are likely responsible for several blasts inside the Demilitarised Zone.
Three South Korean soldiers were injured.
South Korea has vowed to "take appropriate and corresponding measures" if the investigation finds North Korea was responsible.
North Korean landmines are likely responsible for a blast that injured three South Korean military officers inside the demilitarised zone (DMZ), according to Seoul's military.
Three South Korean soldiers were hurt in the explosions last week while clearing a patrol route in the western sector of the DMZ, a heavily fortified area bordering North Korea.
The DMZ is a 248-kilometre long, four-kilometre wide buffer zone that has divided the Korean Peninsula since the 1953 armistice.
But "recently planted mines" by North Korea were believed to have caused last week's explosion, according to preliminary findings from a joint probe between South Korea's Joint Chiefs of Staff and the United Nations Command.
"We have confirmed that the site was about 10 metres from an area where changes in the terrain were observed and where North Korean forces are believed to have recently planted mines," Kwon Dae-won, vice chair of South Korea's Joint Chiefs of Staff, said.
"Taking into account the crater and the pattern of the flames from the explosion, the extent of the injuries sustained by the service members… we assess that North Korean landmines are highly suspected to have caused the blast."
Mr Kwon said if the investigation concluded that Pyongyang was responsible, it "would bear full responsibility for the incident".
"Our military will take appropriate and corresponding measures," he said, without giving details.
Mr Kwon said two landmines exploded and a third, unexploded mine was found while soldiers were withdrawing after the blast on September 21.
While the zone itself is officially demilitarised, its northern and southern boundaries are among the world's most heavily fortified frontiers and remain littered with landmines.
With no peace treaty signed, Seoul and Pyongyang technically remain at war.