Panama's president says his country has seized a North Korean-flagged ship carrying "undeclared military cargo".
President Ricardo Martinelli said the ship, stopped 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 US "commended" Panama for its actions, and said it strongly supported a full inspection of the ship.
The ship, the Chong Chon Gang, was stopped near Manzanillo on the Atlantic side of the canal last week.
It had left Russia's far east in April and travelled across the Pacific Ocean before entering the canal at the start of June, with Cuba as its stated destination.
The ship had crossed the Pacific without its automatic tracking system switched on - a move described by the BBC's security correspondent Frank Gardner as highly suspicious.
Panama's Security Minister Jose Raul Mulino told the BBC that the ship - carrying 250,000 bags of sugar - was seized on 10 July after a tip-off that it was linked to drugs, but the "resistance and violence from the crew" delayed the search.
He said the suspected weaponry was found in two containers and did not rule out further "surprises" as the search of the ship continues.
Panama would be seeking "friendly governments" with the "technology to look into it" to help identify the exact nature of the cargo, he said, adding that there was no contact with North Korea.
Earlier, President Martinelli told local 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."
Neither Cuba nor North Korea has yet 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.