South Korea says it fired warning shots at a North Korean patrol boat that temporarily crossed the countries’ disputed western sea boundary while chasing an unarmed North Korean vessel.

The North Korean patrol boat crossed the so-called Northern Limit Line on Tuesday morning while pursuing the vessel in waters near South Korea’s Baekryeong island and retreated after a South Korean naval ship fired warning shots, according to Seoul’s Defense Ministry and Joint Chiefs of Staff.

South Korean military officials seized the North Korean vessel being chased by the patrol boat and were questioning its seven crewmembers.

Also Read | North Korea conducts ‘important’ satellite test after missile firing

South Korea’s navy has often fired warning shots to repel North Korean vessels crossing the countries’ poorly marked sea border, but there also have been some deadly clashes over the years. South Korea blamed North Korea for an attack on a South Korean warship that killed 46 sailors in 2010, but the North has denied responsibility.

South Korea has patrolled waters around the Northern Limit Line for decades after it was drawn up by the U.N. command at the end of the 1950-53 Korean War. North Korea does not recognize the line and insists upon a boundary that encroaches deeply into waters currently controlled by the South.

Tuesday’s incident came amid renewed tensions created by North Korea’s accelerating missile tests this year, including a launch on Saturday that the North said tested technologies for a spy satellite it is developing.

It came a day before South Koreans vote in a highly contested presidential election. The bitter campaign has been marked by clashes between major candidates over whether South Korea should continue pursuing engagement with rival North Korea or take a harder line to check its nuclear threat.

Also Read | North Korea says it tests systems for spy satellite

Nuclear negotiations between the United States and North Korea have been stalled for three years because of disagreements over a relaxation of crippling U.S.-led sanctions on North Korea in exchange for steps toward its nuclear disarmament.