Authorities in Panama said they intercepted a suspicious North Korean-flagged vessel as it tried to traverse the waterway and found “undeclared military cargo” hidden in shipments of sugar.

The Chong Chon Gang had sailed from Cuba and was approaching the Panama Canal when Panamanian marines detained the vessel at the port of Manzanillo suspecting it was carrying drugs.

The 35 North Korean crew aroused more suspicion when they resisted Panamanian Marines’ efforts to board the vessel. The captain reportedly attempted to commit suicide, claiming he had a heart attack when the vessel was stopped.

Ricardo Martinelli, Panama’s president, inspected the vessel himself on Monday night alongside security forces and said “sophisticated missile equipment” was found hidden beneath more than 200,000 sacks of Cuban sugar.

Panama had been tracking the ship since last Wednesday following a tip-off that it was carrying drugs or arms, reports the Financial Times.

The weapons found on board “appears to be much more than a conventional weapon,” Jose Raul Mulino, Panama’s security minister said.

Martinelli posted a photo on his Twitter account showing what appeared to be a green tubular object, which was later identified by defence consultancy IHS Jane’s Intelligence as an SNR-75 “Fan Song” fire control radar for the SA-2 family of surface-to-air missiles.

“One possibility is that Cuba could be sending the system to North Korea for an upgrade,” IHS Jane’s said.

The seizure is a potential breach of UN sanctions on North Korea and spurred renewed focus on North Korean-Cuban links, said Hugh Griffiths, an arms trafficking expert with Stockholm International Peace Research Institute.

“You cannot go around shipping undeclared weapons of war through the Panama Canal,” Martinelli said.