It came in the form of a diplomatic hand grenade from Washington, just as South Korea was struggling to contain the damage from its earlier admission of secret nuclear research.
Seoul says only miniscule amounts of nuclear material were produced
A US official was quoted as saying that Seoul had experimented with plutonium in addition to the uranium tests already disclosed.
Both materials can be used to make a nuclear bomb.
The Science Ministry in Seoul confirmed at a hastily-arranged news conference that scientists had secretly reprocessed plutonium in the early 80s.
"No records have been left to tell the exact amount of plutonium produced, but we presume a miniscule amount of plutonium may have been extracted," said Kim Young-shik, director-general for nuclear safety.
He said that South Korea was abiding by all its obligations under the nuclear non-proliferation treaty.
South Korean FM Ban Ki-moon says Seoul is co-operating with the IAEA
That is not a view shared by analysts and diplomats, who say there have been at least two clear violations of the nuclear safeguards agreement.
"Taken together this looks like a pattern," said a Vienna-based diplomat who declined to be identified.
South Korea stunned the region on 2 September when it revealed that scientists conducted secret tests to enrich a small amount of uranium in 2000.
It says the experiments were not authorised by the government and were conducted by a small group of scientists out of academic curiosity.
"The experiment had nothing to do with our nuclear programme," said Dr Chang In-soon, the president of the Korea Atomic Energy Research Institute, who gave the go- ahead for the tests.
He said the equipment was dismantled immediately after the research - but conceded that it could have taken several months.
Key questions remain about the enrichment level of the uranium and how long the project continued.
"It's a question of confidence and trust," said the diplomat in Vienna, "Iran changed its story and undermined confidence, and now South Korea is going down the same road."
In its first public comments, North Korea warned of a nuclear arms race in north-east Asia.
A North Korean diplomat at the United Nations also accused the US of double standards.
Washington accuses North Korea of conducting a secret uranium programme in addition to its much better known plant at Yongbyon, that can produce plutonium for atomic bombs.
The US said South Korea should not have conducted the tests but praised it for co-operating with the International Atomic Energy Agency.
Japan has taken a stronger public position.
"It was inappropriate," said Chief Cabinet Secretary Hiroyuki Hosoda, who called for strict inspections. "We must not allow this to lead to development of nuclear weapons," he said.
Diplomats in the region are now braced for damaging repercussions.
South Korea has been at the forefront of efforts, with the United States, to persuade North Korea to abandon its weapons programme, which analysts suspect may include up to six bombs.
Seoul has a lot of explaining to do to regain the trust of the outside world in its often repeated commitment to a nuclear free Korean peninsula.
It will be even harder to convince the North that its best interests lie in giving up its own nuclear ambitions.