"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," Martinelli said.
The governments of North Korea and Cuba made no public comment on the case.
In early July, a top North Korean general, Kim Kyok Sik, visited Cuba and met with his island counterparts. Cuban Communist Party newspaper Granma said he was also received by President Raul Castro, and the two had an "exchange about the historical ties that unite the two nations and the common will to continue strengthening them."
The meetings were held behind closed doors, and there has been no detailed account of their discussions.
"After this incident there should be renewed focus on North Korean-Cuban links," said Hugh Griffiths, an arms trafficking expert at the Stockholm International Peace Research Institute. Griffiths said his institute told the U.N. this year that it had uncovered evidence of a flight from Cuba to North Korea that travelled via central Africa.
"Given the history of North Korea, Cuban military cooperation and now this latest seizure, we find this flight more interesting," he said. "
The Chong Chon Gang has a history of being detained on suspicion of trafficking drugs and ammunition, Griffiths said. Lloyd's List Intelligence said the 34-year-old ship, which is registered to the Pyongyang-based Chongchongang Shipping Company, "has a long history of detentions for safety deficiencies and other undeclared reasons."
Satellite tracking records show it left the Pacific Coast of Russia on April 12 with a stated destination of Havana, then crossed the Pacific and the Panama Canal on its way to the Caribbean. It disappeared from satellite tracking until it showed up again on the Caribbean side of the canal, on Friday, Lloyd's said.
The disappearance from satellite tracking indicates that the crew may have switched off a device that automatically transmits the ship's location after it moved into the Caribbean, Lloyd's said.
Griffiths said the Chong Chon Gang was stopped in 2010 in the Ukraine and was attacked by pirates 400 miles off the coast of Somalia in 2009.
Griffiths' institute has also been interested in the ship because of a 2009 stop it made in Tartus -- a Syrian port city hosting a Russian naval base.