South Korean security forces are on high alert for any attempt by North Korea to wreck the G20 summit of world leaders.
They fear that their neighbour could try to invade some of their territory, test nuclear weapons or attack a South Korean ship.
There were even unsubstantiated reports that the repressive regime was considering biochemical attack against the South by sending deadly materials attached to balloons and parachutes across the border.
Although the North has a track-record of bellicose pronouncements, the British delegation at the summit in Seoul - just 50 miles from the border with North Korea - is taking the threat seriously.
They say the danger is heightened by the process underway in the North of handing over power from Kim Jong-il to his son Kim Jong-un, who is understood to have been charged with drawing up ideas to disrupt the G20 meeting.
British officials believe the country's leadership will want to make a dramatic show of power to demonstrate it is "business as usual" for the regime.
Hillary Clinton, the US Secretary of State, has also warned North Korea not to take any "provocative" actions during today's summit.
A UK diplomatic source said: "There has been some speculation that North Korea would try to disrupt the summit.
"There are a range of things that they could potentially do, such as making an incursion into South Korea or testing something in their own air space that would cause us concern.
"They are in the midst of a succession. The leadership must want to demonstrate it's more than business as usual."
However a UK embassy source in Seoul said the presence of China - regarded as sympathetic to the regime - would deter the North from taking action.
Tensions between the neighbours have further intensified since March when a South Korean warship, the Cheonan, was sunk apparently by a North Korean torpedo.
The year before Seoul hosted the 1988 Olympic games, North Korean agents planted a bomb on a South Korean plane, killing all 115 people on board.
Improving relations on the Korean peninsula and reining in the nuclear ambitions of the North are being discussed by world leaders at the G20 summit. Arriving for the meeting, the US President Barack Obama condemned North Korea for pursing a "path of confrontation and provocation" that, he said, included its relentless pursuit of nuclear weapons.
The South Korean government has mounted a huge security exercise to protect the summit headquarters from protesters.Reuse content