North Korea has urged the Panama government to release its ship that was seized after sophisticated weapons were discovered on board.

Demanding the ship to be released without any delay, the Pyongyang government said that the weapons on board were "old and obsolete" Cuban arms, which were being sent for repairs, adding that the cargo would be sent "back to Cuba after overhauling them, according to a legitimate contract."

Panama earlier this week seized the ship while it was heading toward North Korea from Cuba.

"The Panamanian investigation authorities rashly acted 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," said the North Korean foreign ministry, according to CNN.

After urging the United Nations to investigate the cargo and see if there was any breach of sanctions against the Pyongyang government, Panamanian Foreign Minister Fernando Nunez said North Korea was welcome to visit Cuba to investigate the vessel being held by his government.

 "We've just authorized two visas for representatives of the Republic of Korea, through our embassy in Havana ... We have (provided them) so that they may come with an eye toward providing explanations and inspecting their ship," said the foreign minister in an official statement. 

The North Korean ship was initially stopped on the Atlantic coast of Panama on suspicion of carrying drugs but later it was found to have other cargo of greater concern, according to the President of Panama, Ricardo Martinelli.

"When we started to unload the shipment of sugar, we located containers that we believe to be sophisticated missile equipment, and that is not allowed," said President Martinelli.

"The world needs to sit up and take note: you cannot go around shipping undeclared weapons of war through the Panama Canal," said Ricardo.