Aug. 10, 2015: South Korean protesters stage a rally demanding peace and reunification of the Korean peninsula in Seoul. South Korea says North Korean soldiers placed the two mines that exploded last week at the border and maimed two South Korean soldiers. The letters read 'The improvement of North Korean's relationship' and 'No War and Peace.' (AP Photo/Ahn Young-joon)

South Korea warned North Korea Monday that Pyongyang would face a "pitiless penalty" after it blamed the Communist nation for laying two land mines that maimed two South Korean soldiers last week.

The mines exploded Aug. 4 in the Seoul-controlled southern part of the heavily-fortified Demilitarized Zone (DMZ). The two wounded soldiers had been on a routine patrol at the time. One of the injured lost both legs, while the second lost one leg.

South Korea's Defense Ministry said Monday that it believes North Korean soldiers secretly crossed the border and laid the mines because the splinters from the explosions were from wood box mines, which are used by North Korea. South Korean authorities claimed that the mines were laid around a door on the South Korean side of the border that opened onto the DMZ.

South Korea's Joint Chiefs of Staff issued a statement urging North Korea to apologize and punish those responsible for the mine explosion. There was no immediate official response from Pyongyang.

If North Korea is confirmed to be behind the mine explosion, that would be a violation of the armistice that halted fighting in the Korean War. North and South Korea are still technically at war, as the two nations have never signed a peace treaty.

South Korea's Yonhap News Agency reported that South Korean investigators had determined that the explosion was intended to undermine joint military exercise by U.S. and South Korean troops that are scheduled to take place next week.

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