The declaration came late yesterday -- a day after Panama said it had discovered military equipment, which it believed to be missiles, after impounding the vessel and conducting a drugs search.
Panama has urged UN inspectors to scrutinise the cargo, which could constitute a violation of the strict arms sanctions imposed on North Korea over its nuclear program.
However Cuba, one of North Korea's few allies, claimed the shipment as its own, with the foreign ministry listing 240 metric tons of "obsolete defencive weapons," including two anti-aircraft missile systems, as being on board.
"The agreements subscribed by Cuba in this field are supported by the need to maintain our defencive capacity in order to preserve national sovereignty," the ministry said in an English-language statement.
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Panama President Ricardo Martinelli tweeted a photo of the haul, which experts earlier yesterday identified as an ageing Soviet-built radar control system for surface-to-air missiles.
Martinelli's government said the munitions were hidden in a shipment of 100,000 kilograms of bagged sugar aboard the North Korean-flagged Chong Chon Gang.
South Korea welcomed the seizure.
"If the shipment turns out to be in breach of UN resolutions, we expect the UN Security Council's sanctions committee to take relevant steps expeditiously, said a foreign ministry official in Seoul.
The United States also hailed the discovery.