New information about North Korean freighters stopping in Cuba reveal a rise in trade between the two Communist nations amid last week’s scandal over smuggled weapon parts aboard a ship flagged under the Asian nation.

Shipping monitors indicated Wednesday that at least five North Korean freighters made port of call in Cuba over the last four years, with other ships from Pyongyang believed to have visited the island under different flags or ownership documents, The Miami Herald reported.

“The trade numbers are fuzzy but clearly there’s been more contact between the two countries in recent years,” Michael Madden, editor of the North Korea Leadership Watch, told the Herald.

Discovered hidden under 220,000 sacks of sugar on the ship called the Chong Chon Gang were aging anti-aircraft missiles, two Soviet-era Russian-made MIG-21 fighter jets and 15 plane engines. The ship, which sailed Cuba en route to Panama, was seized a week ago after officials discovered weapons that Cuban officials were "obsolete" and had been shipped out to be repaired and returned to the island.

Panamanian authorities, including President Ricardo Martinelli, toured the boat and expressed their anger and dismay that the planes and weapons were attempting to cross through the Panama Canal.  

"You are all here and are sensing the strong odor of fuel, to such a degree that no one can know what danger Panama was put in,” Panama's Attorney General Ana Belfon told the BBC of the gasoline smell that emanated from the plane’s fuselages.

The government has said a team of technical experts from the United Nations is expected in Panama during the first week in August to inspect the cargo and determine if it violates the arms embargo currently in place against North Korea.

The ship's crew has been charged with violating the country's national security, transporting military equipment and not declaring it.

The North Korean captain of the ship tried to commit suicide when discovered and 35 crewmen were arrested after resisting police efforts to intercept the ship.

Under current sanctions, all U.N. member states are prohibited from directly or indirectly supplying, selling or transferring arms, missiles or missile systems and the equipment and technology to North Korea, with the exception of small arms and light weapons.

The ship’s circuitous and mysterious route to and from Cuba has also given pause to some experts, especially when the Chong Chon Gang turned off its location transponder when it entered the Caribbean in early June.

What is known of the ship’s voyage is that it left Nakhodka in the Russian Far East – about 125 miles east of the North Korea border – on April 12 and made it to the Pacific entrance of the Panama Canal on May 31. Since entering the Caribbean on June 1 until it attempted to re-cross the Canal loaded with the weapons on July 11 there is no information of its movement and thus way to determine where the ship stopped in Cuba.

The other North Korean ships to stop in Cuba had all made port at the sugar-exporting port of Puerto Padre, with forays to Havana and Santiago de Cuba as well.

All five North Korean ships were owned by Ocean Maritime Management (OMM), a state shipping company based in Pyongyang, and operated in “a classic shell company network,” according to the Wisconsin Project on Nuclear Arms Control, which monitors international shipping reports.

Matthew Godsey of the Wisconsin Project told the Miami Herald that these stops by North Korean ships might only be the tip of the iceberg as these records don’t include visits by ships flagged or owned outside of North Korea. Under so-called "flags of convenience,” countries and owners can register their ships in other countries to avoid law enforcement oversight or hide their identity.

The Associated Press contributed to this report.

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