Investigators unloading the cargo of a seized North Korean ship that carried arms from Cuba may have discovered explosive material on board, a senior Panamanian official said on Friday.

Javier Caraballo, Panama's top anti-drug prosecutor, said officials removed three more containers from the Chong Chon Gang, but had decided not to open them for "security reasons."

"Experts in the matter tell us that they could contain some type of explosive material," Caraballo told reporters.

He said the Panamanian government would not speculate on the contents until they had been opened. Explosive experts would arrive on Saturday to determine the content of the containers and could announce their findings as early as Sunday, he added.

Caraballo declined to provide further details on what led authorities to believe it could contain explosives.

Panama stopped the ship last week and seized its cargo after a standoff with the North Korean crew.

After Panama discovered weapons hidden beneath 220,000 sacks of sugar, Cuba said the cargo also contained "obsolete" Soviet-era weapons including missile equipment, MiG fighter jets and other arms being sent to North Korea for repair. - Reuters