South Korea says it has persuaded China and Japan to take part in financial sanctions against North Korea.
It comes in the wake of the sinking of a South Korean naval ship by a North Korean submarine.
In a BBC interview, South Korean Foreign Minister Yu Myung-hwan speculated that the sinking of the Cheonan might have been the result of internal Pyongyang politics.
North Korea has angrily denied any responsibility for the attack.
The North Korean military, Mr Yu said, might have decided to do it in order to distract attention from other problems.
Mr Yu had no criticisms to make of China for failing to condemn North Korea publicly for the sinking of the Cheonan.
On the contrary, he felt that China had been privately helpful and understanding during the three-way summit with South Korea and Japan, which ended on Sunday.
South Korea is hoping to make the regime in Pyongyang pay a heavy price for its actions.
It is invoking UN resolution 1695, passed in 2006, which makes it possible to introduce economic sanctions against North Korea for breaching its international obligations.
The government in Seoul wants all inflows of money to North Korea to be stopped.
If China, which has so far delayed announcing its own findings on the sinking of the Cheonan, does take part in the sanctions, it could make life even harder for President Kim Jong-il's regime.
Things are already difficult enough for him. The famine of the past seems mostly to have been dealt with, at least for now, as a result of the government's decision to allow a quiet measure of free enterprise.
But this has helped to create a great deal of corruption among low-level officials, on a scale which the North Korean regime had never previously allowed.
Towards the end of last year North Korea introduced a currency reform which had the effect of raising some salaries by as much as 10,000%.
This made it possible for a lot of people to buy things they had only previously dreamt of.
The shops were suddenly filled with great quantities of cheap consumer goods, mostly brought or smuggled in from China. But it also introduced alarming rates of inflation.
The inflow of cash from outside has never been large, but it has helped to keep North Korea's immensely fragile economy going. If the Chinese join in a wider blockade, it could have serious political consequences as well as economic ones.
It comes at a time when Kim Jong-il, the so-called "Dear Leader", is becoming manifestly weaker. He was once surprisingly clever at wrong-footing both South Korea and the United States in the past, but ever since his stroke in 2008 his powers have waned.
Now his third and youngest son, Kim Jong-un, seems to have been selected to take over from him, just as Kim Jong-il himself was once chosen to succeed "the Great Leader", Kim Il-sung.
Scarcely anything is known about Kim Jong-un, who is either 27 or 28 - accounts vary. He is thought to have been educated in Switzerland, and he possibly speaks quite good English.
Only one authentic photograph of him exists in the public domain, and since that was taken when he was eleven it is not much of a guide to his appearance now.
Rumour in South Korea has it that he was involved in the disastrous decision to reform the currency last year.
Mr Yu admitted in his BBC interview that all these things made people feel nervous about the future of the peninsula at present.
"If things go wrong it could have very serious consequences far wider than just Korea," he said.
It sounds like a measured understatement.