South Korea's president has asked the country's northern neighbour not to cancel reunions of families divided by the war.
North Korea has threatened to do so in protest against upcoming military exercises between the US and South Korea.
The US military has reinforced its presence in the country with an armoured cavalry battalion of 850 soldiers.
Seoul pays $850m annually for US military assistance as an added deterrent to North Korea.
Al Jazeera's Harry Fawcett reports from the port of Busan.