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.

The firing heightens animosities between the rival Koreas, even as North Korea’s tensions with the United States seem to have eased after the release of two detained Americans a day earlier.

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 because of office policy. There were no reports of casualties.

South Korean troops have detected North Korean soldiers approaching the border several times this year, but this was only the second time that the South fired warning shots, the officials said.

The incident happened inside the 4-kilometer (2.5-mile) -wide Demilitarized Zone that was created when the 1950—53 Korean War ended with an armistice.

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

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

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

The conflict over the leafleting appears to have derailed talks on a high-level meeting between the two Koreas, which had been offered by North Korea during the September Asian Games and had been discussed for October or November.

Over the weekend, North Korea released the last two Americans it has been detaining Matthew Miller of Bakersfield, California, and Kenneth Bae of Lynnwood, Washington in a move widely seen as an attempt to break a long deadlock with the U.S. over talks on the North’s nuclear program.

The Korean Peninsula remains in a technical state of war because the Korean War armistice has never been replaced with a peace treaty.