Photo/Illutration A North Korean flag flutters in the wind atop a 160-meter tower in North Korea's Gijungdongseen village, as seen from the Taesungdong freedom village inside the demilitarized zone in Paju, South Korea, on April 27, 2018. (AP file photo)

SEOUL--South Korea’s military fired warning shots Wednesday to repel a North Korean merchant ship after it violated their disputed western sea boundary, Seoul officials said, the second such incident in two months.

Seoul’s Joint Chiefs of Staff said in a statement that it believes the North Korean ship crossed the sea boundary due to bad weather and an engine problem. No further violence was reported.

It’s the second time that South Korea has fired warning shots to drive back a North Korean ship in the area since South Korea’s current liberal government took office in 2017. The first incident happened in September, according to South Korea’s military.

Ties between the two Koreas are strained amid a stalemate in U.S.-led diplomacy on ending the North Korean nuclear crisis.

North Korea said Monday its troops conducted artillery firing drills near the sea boundary. South Korea on Tuesday sent a formal protest message to North Korea, saying the drills violated last year’s inter-Korean agreements aimed at reducing front-line military animosities.

The sea boundary is a source of tension between the two Koreas. They have fought several deadly naval skirmishes in the area in past years. In 2010, two attacks blamed on North Korea near the sea boundary killed 50 South Koreans.