Pirates have seized two ships, one registered in Greece and the other in Hong Kong, off the coast of Somalia.
The Greek ship, reportedly taking a cargo of salt to Kenya, was boarded off Somalia's south coast and is now said to be bound for the Puntland region.
An analyst said the raid showed pirates were expanding their zone of operations southwards from the Gulf of Aden.
Pirates based in war-torn Somalia regularly attack vessels plying major commercial shipping routes nearby.
The fate of the 25 crew aboard the Greek ship - said to be of Filipino origin - is not known.
Reports say the ship, named the Centauri, may now be heading for the port of Eyl.
"The ship was expected to call at Mombasa on 19 September and discharge 17,000 tonnes of bulk salt," Andrew Mwangura of the Kenya-based East African Seafarers' Association told the Reuters news agency.
"It was taken in southern Somalia, which is unusual for the pirates. They could be taking it to Eyl," he added.
A ship registered in Hong Kong was also hijacked off Somali waters late on Wednesday, Mr Mwangura said.
The Great Creation was en route from India to Tunisia, with 24 Chinese crew and one Sri Lankan aboard.
Pirates have seized more than 30 ships off Somali waters this year, Mr Mwangura said.
Several vessels are being held for ransom at Eyl.
Earlier this week, French commandos rescued two sailors amid fears they were being led to a pirate stronghold at the port.
The head of the Malaysia-based International Maritime Bureau's Piracy Reporting Centre told the Associated Press the attack on the Greek boat showed that Somali pirates had expanded their area of operations southwards from the Gulf of Aden.
A multinational naval force, headquartered in Djibouti, has been patrolling the Gulf of Aden against pirates.
Somalia has been without a functioning central government for 17 years and has suffered from continual civil strife.