(BBC) A South Korean company illegally sold more than 100 tonnes of a potentially lethal chemical to the North last year, according to South Korean officials.
They said sodium cyanide was illegally exported via China to a North Korean trading firm.
The substance normally has agricultural or industrial uses, but can also be turned into a deadly nerve agent.
North Korea, which claims to have nuclear arms, is also believed to hold chemical and biological weapons.
Because of its dual civilian and military use, sodium cyanide is subject to multinational control regimes to which South Korea is party.
The ministry of commerce in Seoul said the South Korean firm was found to have secretly sold the substance, via the Chinese company, between June and September last year.
The shipment was made without the required government approval, and was re-exported from China to North Korea, the ministry statement said.
The head of the South Korean company later received a suspended prison sentence for violating the country’s export laws, the statement added.
No party in the alleged deal was identified.
Sodium cyanide is one component needed to make the nerve agents sarin, that can cause paralysis and death, and tabun.
In 1995, 22 people died when members of a Japanese cult released sarin in the Tokyo underground.
Last week North Korea said it would “never dismantle” its nuclear arsenal while the US remained hostile.
Earlier this month, Pyongyang said six-nation talks on its nuclear disarmament could not continue until South Korea’s secret nuclear experiments were “fully probed”.
South Korea recently disclosed its scientists had violated nuclear accords by producing small amounts of enriched uranium and plutonium.