HAVANA — Cuba’s explanation that it buried antiquated weapons systems under thousands of tons of sugar and sent them back to North Korea for repair is potentially credible but leaves troubling questions unresolved, international arms experts said Wednesday.

Acting on intelligence it hasn’t publicly described, Panama seized the rusting, 34-year-old North Korean freighter Chong Chon Gang on July 11 as it headed toward the Caribbean entrance of the Panama Canal on its way to the Pacific and its final destination of North Korea.

Hidden under some 240,000 white sacks of raw brown Cuban sugar, Panamanian officials found shipping containers with parts of a radar system for a surface-to-air missile defense system, an apparent violation of U.N. sanctions that bar North Korea from importing sophisticated weapons or missiles.

North Korea has not commented on the seizure, in which 35 of its nationals were arrested after resisting police efforts to intercept the ship in Panamanian waters last week, according to the Central American country’s government. The captain had a heart attack and also tried to commit suicide, said Panamanian President Ricardo Martinelli.

Nearly 24 hours after Panama announced the find and said it would continue searching the ship for more contraband, Cuba acknowledged late Monday that the ship’s cargo included 240 metric tons of “obsolete defensive weapons”: two Volga and Pechora anti-aircraft missile systems, nine missiles “in parts and spares,” two Mig-21 Bis and 15 engines for those airplanes. The equipment was meant to be repaired in North Korea and returned to Cuba, the Cuban government said.

North Korea has a robust capability to repair and upgrade Soviet-era military equipment, and the economically struggling, isolated nation has a track record of trading technical help for commodities such as sugar, experts said.

At the same time, North Korea is known to be seeking to evade sanctions and get spare parts for its own weapons systems, particularly Mig jet fighters. That raises the possibility that in lieu of cash, Cuba was paying for the repairs with a mix of sugar and jet equipment, experts said.

“We think it is credible that they could be sending some of these system for repair and upgrade work,” said Neil Ashdown, an analyst for IHS Jane’s Intelligence. “But equally there is stuff in that shipment that could used in North Korea and not going back.”

Britain’s U.N. Ambassador Mark Lyall Grant said Wednesday that “any weapons transfers, for whatever reason, to North Korea would be a violation of the sanctions regime and therefore there are questions to be answered.”