The Mount Kumgang resort had been closed since a North Korean soldier shot a South Korean tourist there in 2008, drying up a lucrative source of hard currency for the North.
After months of threats and counter-threats from both sides, Pyongyang said it would now "legally dispose" of South Korean assets from the resort after Seoul failed to meet Friday's final deadline to agree on asset disposal.
Fourteen South Koreans were listed as being at the east coast resort as of Monday, South Korea's Unification Ministry website said.
The ministry, which deals with inter-Korean affairs, did not provide immediate comment on the North's latest threat.
The resort was built by an affiliate of the South's Hyundai Group in the early 1990s and has been visited by more than a million South Koreans.
North Korea's estimated $17 billion a year economy has been dealt a heavy blow by international sanctions over the past three years, imposed over its reluctance to rein in its nuclear programme, and by a botched currency move at the end of 2009.
Four Hyundai Asan officials travelled to the Mt Kumgang resort on Friday in a bid to end the dispute.
Copyright APP (Associated Press of Pakistan), 2011
Copyright Reuters, 2011