South Korean troops fired warning shots Monday after North Korean soldiers approached too close to the border separating the rival countries, Seoul defense officials said.

About 10 North Korean soldiers retreated without returning fire after South Korean troops fired 20 rounds of warning shots, the officials said on condition of anonymity, in line with office policy. There were no reports of casualties.

The incident happened near the military demarcation line inside the 2.5-mile-wide Demilitarized Zone that was created when the 1950-53 Korean War ended in an armistice.

Tensions between the Koreas remain high following two incidents last month involving exchanges of gunfire, though no casualties were reported in either one.

In the first incident, troops from the two countries traded gunfire over propaganda leaflets South Korean activists floated across the border. In the second, North Korean soldiers were seen too close to the border, triggering a gunfire exchange. The North said its soldiers were engaging in routine patrol missions.

At the center of their recent animosities is North Korea's demand that South Korea ban activists from launching the anti-Pyongyang leaflets. South Korea has said it cannot do so, citing freedom of speech.

The Korean Peninsula remains in a technical state of war because the Korean War armistice has not been converted to a peace treaty.