The demand for the return of the ship Chon Chon Gang and its crew was issued by the North Korean Foreign Ministry, whose spokesman told the North's Korean Central News Agency the vessel was seized even though Panamanian authorities could not find any narcotics as they suspected.
"The Panamanian authorities should release our crew and vessel without delay," the spokesman said.
The 14,000-ton ship was seized by Panamanian authorities in the Panama Canal as it sailed home from Cuba, South Korea's Yonhap News reported.
On Tuesday, the Cuban Foreign Ministry in a statement confirmed the weapons were aboard the vessel along with a shipment of sugar, saying the weapons were obsolete and were being shipped to North Korea to be repaired under a contract.
The Cuban statement said the shipment included "240 metric tons of obsolete defensive weapons -- two anti-aircraft missile complexes Volga and Pechora, nine missiles in parts and spares, two Mig-21 Bis and 15 motors for this type of airplane, all of it manufactured in the mid-20th century."
CNN quoted the North Korean Foreign Ministry spokesman as saying Panamanian authorities "rashly attacked and detained the captain and crewmen of the ship on the plea of 'drug investigation' and searched its cargo but did not discover any drug."
The spokesman said the cargo "is nothing but aging weapons" that were to be overhauled and returned to Cuba under a "legitimate contract."
Panamanian authorities have said the North Korean ship's captain tried to kill himself and crew members resisted arrest after inspectors found the suspected weapons system components.
North Korea is under strict U.N. sanctions for violating its resolutions against conducting nuclear and missile tests and is prohibited from importing or exporting most weapons.
The BBC quoted Panamanian Security Minister Jose Raul Mulino as saying Wednesday authorities had found "two more containers" with suspected arms besides those already confiscated.
Panama has sought guidance from the United Nations on handling the case and expects U.N. officials to visit Panama to investigate, Foreign Minister Fernando Nunez told CNN. The Panamanian government also has filed a report with the United Nations' North Korea sanctions committee.
Panamanian authorities said the ship's captain, who allegedly suffered a heart attack and then tried to commit suicide when the vessel was being searched, and the 35-member crew could be charged with threatening national security, CNN reported.
The report, quoting two U.S. officials, said the United States and Panama had been tracking the North Korean ship after it crossed the Panama Canal to Cuba and then back. The United States also planned to assist in the investigation.
Earlier, U.S. State Department spokesman Patrick Ventrell commended the Panamanian government's action, saying Washington "strongly supports Panama's sovereign decision to inspect the DPRK-flagged vessel." He said Panama is a close partner of the United States.
Ventrell also said "if indeed there were a shipment of arms on board of this vessel, any shipment of arms or related materiel" would be in violation of U.N. Security Council Resolutions 1718, 1874 and 2094 relating to North Korea.
The BBC said the MV Chong Chon Gang had traveled from Russia's Far East in April across the Pacific Ocean before entering the Panama Canal in early June destined for Cuba. The report said the ship crossed the Pacific without turning on its automatic tracking system, raising suspicions.
Some analysts have told CNN the entire incident may indicate North Korea could be supplying Cuba with weapons, while noting the geographical proximity of Cuba to the United States.
In its statement, the Cuban Foreign Ministry said the country remains committed to "peace, disarmament, including nuclear disarmament, and respect for international law."