Somali pirates have hijacked a chemical tanker with 23 crew members on board, soon after releasing another ship for which they received a ransom.

The South Korean foreign ministry says pirates seized the Chemstar Venus and its crew of five South Koreans and 18 Filipinos off the coast of Somalia late Saturday.  The ship is owned by a Japanese company.  There has been no word on the condition of the crew.

The hijacking came shortly after pirates freed another Japanese-owned chemical tanker, the Stolt Valor, on  Saturday.  Indian officials say all crew members aboard that ship are safe, including 18 Indians, two Filipinos, a Bangladeshi and a Russian. 

Officials say a ransom was paid to the pirates, who seized the ship in September.  

In a separate incident, Russia's navy says its forces prevented the seizure of a Saudi-owned vessel during a pirate attack in the Gulf of Aden Saturday.

Russian officials say a navy warship was guarding three cargo vessels through the Gulf when it received a distress call from the Saudi ship, Rabih.  Officials say navy forces repelled the pirates, who were approaching the Saudi ship on speedboats.

International Maritime officials say at leaset 83 have been attacked off Somalia this year, with 33 of them hijacked.  The pirates are currently holding about 11 ships, including a Ukrainian cargo vessel carrying 33 tanks.

Somalia's interim government is fighting a strengthening Islamist insurgency, and does not have forces to patrol its territorial waters.

Some information for this report was provided by AFP, AP and Reuters.