The two Koreas ended the opening day of high-level military talks earlier than expected on Tuesday, as North Korea renewed its long-running demand that the western sea border with the South be redrawn.
The sea border dispute has been widely considered a deal breaker in this week's talks set to run through Thursday.
North Korea does not recognize the current sea border demarcated by the United Nations at the end of the 1950-53 Korean War, and claims the border is too far north.
South Korea has rejected the North's claim, saying the current border should be respected.
"Of course there was a mention of the Northern Limit Line," said Col. Moon Seong-mook, spokesman for the South Korean delegation, referring to the official name of the sea border. "The North said the issue needs to be discussed. We stressed again that our position ... is firm."
The talks began at 10 a.m. (0100 GMT) at the truce village of Panmunjom in the middle of the Demilitarized Zone running between North and South, and concluded three hours later, Moon said.
Despite the early conclusion of the opening session, Moon said the talks' atmosphere was "not bad."
"We ended the talks early because the two sides sufficiently put forward positions ... and decided it would be more effective and productive to go back and review those issues and meet against tomorrow," he said.
This week's talks - the highest-level regular dialogue channel between the two militaries - are aimed at fleshing out agreements reached at their previous session in May. The agreements include setting up a joint fishing area around their disputed maritime border off the peninsula's west coast and preparing security arrangements for joint economic projects near the border.
The two sides have since held three rounds of lower-level talks to discuss the agreements, but no progress was made because North Korea repeated its long-running demand that the sea border be redrawn further south.
North Korea's navy command has issued a series of warnings in recent months that a skirmish along the disputed maritime border in the Yellow Sea - the scene of deadly clashes in 1999 and 2002 - could occur again unless South Korean vessels stop entering the North's waters.
South Korea has rejected the North's position, saying its vessels operate only south of the U.N.-demarcated border.
The waters around the border are rich fishing grounds and boats from the two Koreas routinely jostle for position during the May-June crab-catching season.
The Korean War ended in a cease-fire that has never been replaced with a peace treaty - leaving the two sides technically at war.