Relations between North and South Korea have worsened markedly since the March sinking of the patrol ship Cheonan which left 46 South Korean sailors dead.
An international investigation blamed the sinking on a North Korean torpedo, but Pyongyang has rejected the allegations threatening war if it is punished over the incident.
In a further reflection of tensions between the two Koreas, defence officials in Seoul said on Friday that the North had declared a ban on shipping in the waters off its west coast –a usual precursor to missile tests.
"The move is seen as part of a routine exercise by North Korean troops, but we are closely monitoring their activities," an unnamed spokesman told AFP news agency.
"We have yet to detect any signs of preparations for missile launches," the official said.
Mike Chinoy, an expert on Korea, told Al Jazeera that the Korean peninsula remains one of the most volatile flash points in the world.
"The tensions that remain between the north and the south are largely due to the fact that the war has never officially ended," Chinoy said.
"There was an armistice that brought the shooting to an end in 1953 but technically ... until some kind of final peace agreement is reached this conflict always has the potential to escalate.
"Both sides talk about reunification but the reality is that neither side is interested in it in a real way in the near term."
At Friday's ceremony marking the 60th anniversary of the start of the conflict, South Korea's president presented plaques of thanks to representatives of the 21 nations that sent troops or medical units to support the South under a United Nations flag during the war.
"South Korean and UN soldiers, you were not only courageous and genuine soldiers but also a cornerstone of South Korea's history," Lee said.
"We will remember your sacrifice and dedication forever."
|Al Jazeera's Tony Birtley reports on the memorial for the 'forgotten' Korean war [AFP]
The 1950-53 war, which began with a North Korean invasion, cost close to three million lives by most estimates.
For its part North Korea insists that South Korea and its US ally instigated the war.
"The war was actually kicked off due to provocations from the south," the North's official news agency said Thursday.
"No matter what gimmick the US imperialists may employ, they can never hide their true colours as provokers of the Korean War and aggressors."
The cash-strapped state has demanded that the United States pay almost $65 trillion in compensation for six decades of hostility.
It says the figure includes $26.1 trillion arising from US war "atrocities", alleging that these left more than five million North Koreans dead, wounded, kidnapped or missing.