Panama's president says his country has seized a North Korean-flagged ship carrying "undeclared military cargo".
President Ricardo Martinelli said the ship, held in the Panama Canal as it sailed from Cuba, contained suspected "sophisticated missile equipment".
He posted a photo of what looked like a large green object inside a cargo container on his Twitter account.
The president said the 35-strong crew had resisted the search and the captain had tried to kill himself.
The ship, the Chong Chon Gang, was stopped near Manzanillo on the Atlantic side of the canal.
Mr Martinelli said the authorities were checking the vessel for drugs when they found the suspected weapons in containers of brown sugar.
"We're going to keep unloading the ship and figure out exactly what was inside," he told local media.
Mr Martinelli told RPC radio: "This material not being declared and Panama being a neutral country, a country in peace, that doesn't like war, we feel very worried about this war material and we don't know what else will have... passed through the Panama Canal."
The 35-member crew have been detained, including the captain.
Security Minister Jose Raul Mulino said the ship "aroused suspicion by the violent reaction of the captain and the crew".
Neither Cuba nor North Korea has so far commented on the incident.
Under United Nations sanctions, North Korea is banned from weapons exports and the import of all but small arms.
Sanctions were strengthened after its third nuclear test on 12 February, including measures increasing states' authority to inspect suspicious cargo.
In recent years a number of North Korean ships have been searched under the UN sanctions.
In July 2009 a North Korean ship heading to Burma was tracked by the US Navy on suspicion of transporting weapons and subsequently turned around.
North Korea has an ongoing missile development programme.
In December it launched what it called a three-stage rocket to put a satellite into space. The move was condemned by the UN as a banned test of missile technology.
Experts believe the communist state is working towards developing a nuclear warhead small enough to put on a long-range missile.