Senior army generals from North and South Korea have met for rare talks that could see the first border rail crossings in more than 50 years.
The South hopes to connect its rail lines to the North
South Korea has said it hopes the three-day talks will result in security for a planned test-run on 17 May.
But North generals opened the meeting by raising a sensitive issue, which has overshadowed similar talks in the past.
They said they wanted to discuss their long-standing demand that the western sea border be redrawn.
Pyongyang does not recognise the border line drawn up by the UN at the end of the 1950-53 Korean War, which has often resulted in naval clashes in the disputed rich fishing waters.
Journalists were allowed to cover the opening of the meeting in the truce village of Panmunjom - the first such talks at general-level for a year.
240 km long and 4km wide, the DMZ takes up about 0.5% of Korean peninsula
N Korea has 1.1 million man army along the border. S Korea and US forces total more than 700,000
"Isn't a navy official included in your delegation?" the North's chief delegate Kim Young-choi asked his South Korean counterpart as talks opened.
He went on to say that his delegation had never agreed that the railway crossings would be the only item on the agenda.
"We came to today's talks for the issue of preventing naval clashes in the West Sea and realising a joint fishing area," he said.
A South Korean defence ministry official told Yonhap news agency that Pyongyang's stance was not unexpected.
"As you know, North Korea likes to start negotiations with maximum demands," he said.
Seoul hopes the talks will result in a pledge by the North to ensure the security of a test rail crossing across the heavily-fortified border on 17 May.
It would be the first such crossing since the border was sealed at the end of the war.
A planned trial crossing a year ago was cancelled by North Korea, reportedly because the South refused to discuss the sea border issue.
Although the two sides are still technically at war - the peace treaty to officially end it was never signed - links have slowly been re-established in recent years as the South has sought to engage with its secretive neighbour.
Railway tracks have already been laid both on the east and west sides of the border, but they have yet to be tested.
If and when the rail link finally opens, there are obvious advantages for both sides.
For South Korea, it would mean that goods produced using cheap North Korean labour and land, in a joint industrialised zone to the north of the border, could be more easily transported out of the country.
For the North, it would boost the tourist trade, providing a link to a mountain resort on the east coast popular with South Koreans.