Cuba announced Tuesday that weapons found on a North Korean ship close to the Panama Canal were "obsolete" Soviet-era arms, which the communist island had sent to Pyongyang for repair.
The declaration came a day after Panama said it had found military equipment, which it believed to be missiles, after impounding the ship and conducting a drugs search.
North Korea on Wednesday insisted that Panama free the ship. "This cargo is nothing but aging weapons which are to send back to Cuba after overhauling them according to a legitimate contract," the nation's official KCNA news agency cited a Foreign Ministry spokesman as saying, according to Reuters.
"The Panamanian authorities should take a step to let the apprehended crewmen and ship leave without delay," the foreign ministry urged, reported the Associated Press.
Earlier Tuesday, Panama called for United Nations inspectors to scrutinize 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 defensive weapons," including two anti-aircraft missile systems as being on board.
There were also "nine missiles in parts and spares" various Mig-21 aircraft parts and 15 plane motors, "all of it manufactured in the mid-20th century" and "to be repaired and returned to Cuba."
"The agreements subscribed by Cuba in this field are supported by the need to maintain our defensive capacity in order to preserve national sovereignty," the ministry said in an English-language statement.
Panama President Ricardo Martinelli tweeted a photo of the haul, which experts earlier Tuesday 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 220,000 pounds (100,000 kilograms) of bagged sugar aboard the North Korean-flagged Chong Chon Gang.
Panama's Security Minister Jose Raul Mulino told RPC radio that the affair now is a matter for UN investigators.
The United States hailed the discovery.
"We stand ready to cooperate with Panama should they request our assistance," State Department spokesman Patrick Ventrell said, reiterating that any shipments or "arms or related material" would violate several Security Council resolutions.
The magazine IHS Jane's Defence Weekly said Tuesday that the photo tweeted by Martinelli appeared to show an "RSN-75 'Fan Song' fire-control radar system."
The weapons were developed in 1957 and frequently used during the Vietnam War.
Panamanian officials said Monday that the crew resisted the search last Friday, and that the ship's captain attempted to commit suicide after the vessel was stopped.
It was sailing from Cuba towards the canal with a crew of about three dozen and was stopped by drug enforcement officials and taken into port in Manzanillo.
A Panama government spokesman said an examination of the ship by weapons specialists may take as long as a week.
"The world needs to sit up and take note: you cannot go around shipping undeclared weapons of war through the Panama Canal," Martinelli told Radio Panama listeners on Monday.
The vessel was being held in a restricted zone, and the crew had been detained, officials said. So far, no drugs have been found on board.
North Korea's army chief of staff General Kyok Sik Kim visited Cuba last month and said the two countries were "in the same trench."
North Korea carried out a third nuclear weapons test in February, triggering tighter UN sanctions.
Experts say it is unclear whether the North has the technology to build a nuclear warhead for a missile.
UN sanctions bar the transport of all weapons to and from North Korea apart from the import of small arms. Several of the country's ships have been searched in recent years.
In July 2009, a North Korean ship heading to Myanmar, the Kang Nam 1, was followed by the US Navy due to suspicions it was carrying weapons. It turned around and headed back home.
Pyongyang has yet to comment on the latest incident.
Five percent of the world's commerce travels through the century-old Panama Canal, and that is expected to increase following the completion of a major expansion project.