The North and South Korean navies have established radio contact for the first time, as moves to reduce tensions along their Cold War border gather pace.
The two sides clashed in 2002
Warships exchanged brief messages to say neither had hostile intentions.
Allowing radio contacts was one of several measures agreed earlier this month by the two countries' militaries to stop any repeat of border clashes.
But North Korea has remained defiant over international calls that it should scrap its nuclear programme.
The country's state media criticised last week's statement by the G8 nations, claiming the group was trying to spark another Iraq crisis by imposing nuclear inspections.
Known as Northern Limit Line
Position declared by UN in 1953
Not recognised by North
Regularly breached by North's fishermen
The two militaries met earlier this month and agreed a number of measures to ease tensions, including a telephone hotline between the rival navies, the sharing of radio frequencies and the use of visual signals.
The agreement was important because the naval border is still disputed, which led to gun battles in 1999 and 2002.
Monday's radio exchange was welcomed by South Korea's navy.
"With the successful communication today, I believe that the dangers of accidental clashes taking place along the western sea will be reduced," said Lt Commander Yoo
Following the exchange, the two sides are also due to stop broadcasting propaganda across the border and start taking down signs along the Demilitarised Zone (DMZ).
The measures are due to be in place by 15 August, the anniversary of Korea's 1945 independence from Japanese colonial rule.
Correspondents say that although North Korea appears keen to improve ties with the South, it has not shifted position on its nuclear ambitions.
A third round of six-nation talks on the nuclear stand-off are due to convene in Beijing this month, but neither the US nor North Korea has shown any sign of urgency to resolve the issue.