North Korea has warned that the Korean peninsula was headed for "thermo-nuclear" war and advised foreigners in South Korea to consider evacuation.
In the latest in a series of threats, Pyongyang said on Tuesday that it could not ensure the safety of their personnel if a conflict broke out.
"The situation on the Korean Peninsula is inching close to a thermo-nuclear war," the North's official news agency said.
Saying it did not want to see foreigners in South Korea "fall victim", the statement requested all foreign institutions, enterprises and tourists "to take measures for shelter and evacuation in advance for their safety".
The North blamed the heightened war risk on the "warmongering US" and its South Korean "puppets" who were intent on invading the North.
The US dismissed the warning as "unhelpful" on Tuesday.
White House spokesman Jay Carney criticised Pyongyang for more "unhelpful rhetoric that serves only to escalate tensions".
The threat comes as South Korea increased its surveillance of its northern neighbour, raising its "Watchcon 3" status, a normal defence condition, by one level in order to step up monitoring and increase the number of intelligence staff, a senior military official told the Yonhap news agency on Wednesday.
The "thermo-nuclear war" threat has been wielded several times in recent months, most recently on March 7, despite expert opinion that the North is nowhere near developing such an advanced nuclear device.
"It is our current assessment that there is no immediate risk to British nationals in South Korea," a British embassy spokesman said, echoing similar statements from the US, French and other missions.
Last week's warning to embassies was also largely dismissed as empty rhetoric, with most governments involved making it clear they had no plans to withdraw personnel from their Pyongyang missions.
"It's almost comic," Daniel Pinkston, a North Korea expert with the International Crisis Group, said of the latest threat.
"They want to rattle the investment market, create pressure and make people nervous.
"But it's just not working. It's as if they didn't get a rise out of the embassies in Pyongyang, so they're just moving on to the next target," Pinkston said.
The South Korean stock market closed slightly up on Tuesday, before the statement was published.
The Korean peninsula has been locked in a cycle of escalating military tensions since the North's third nuclear test in February, which drew toughened UN sanctions.
A top US military commander said Tuesday that he favoured shooting down a North Korean missile only if it threatened the United States or Washington's allies in the region.
When asked by lawmakers if he supported knocking out any missile fired by North Korea, Admiral Samuel Locklear, head of US Pacific Command, said: "I would not recommend that".
But he told the Senate Armed Services Committee he would "certainly recommend" intercepting an incoming North Korean missile "if it was in defence of our allies" or the United States.
Pyongyang's bellicose rhetoric has reached fever pitch in recent weeks, with near-daily threats of attacks on US military bases and South Korea in response to ongoing South Korea-US military exercises.
There has been no significant foreign investment fallout in South Korea from the current crisis, and Tuesday's threat was unlikely to cause any great consternation for a foreign community of around 1.4 million that has calmly weathered the rhetorical storm thus far.
Earlier Tuesday, North Korean workers followed Pyongyang's order to boycott the Kaesong joint industrial zone with South Korea, signalling the possible demise of the sole surviving symbol of cross-border reconciliation.
"Not a single worker showed up this morning," the manager of one of the 123 South Korean companies operating in Kaesong told reporters.
The North announced Monday it was taking the unprecedented step of pulling out its 53,000 workers and shutting the complex down indefinitely.
Established in 2004, Kaesong has never been closed before. Pyongyang's move reflects the depth of the current crisis on the Korean peninsula, which has otherwise been more notable for fiery rhetoric than action.