The Taiwanese ship MV Win Far 161 was seized near an island in the Seychelles in the Indian Ocean, the diplomats said. It is the second attack in the Seychelles within a week.
No location was given for the attack on the British-owned cargo ship the Malaspina Castle. The ship is Italian-operated and flies a Panamanian flag.
There was no word on the nationalities or numbers of crew on board the two vessels or the exact time of the attacks.
The diplomats spoke on condition of anonymity because they were not authorized to speak to the media.
The hijacking of the Taiwanese ship is the latest in a series of attacks in the Indian Ocean. Analysts say the pirates have moved many of their operations out of the Gulf of Aden, which is heavily patrolled by naval warships from countries as diverse as China, the United States, France and India. Instead, they are attacking off the east African coast, targeting ships coming out of the Mozambican channel.
The multimillion-dollar ransoms are a rare source of cash in , where nearly half the population is dependent on food aid and clan-based militias are tearing the country apart. The lawless Horn of Africa nation has not had a functioning government since 1991.