The St James Park issued a distress call on Monday night saying it had been attacked by pirates
Somali pirates have captured two ships with 45 crew off the East African coast, officials say.
A UK-flagged chemical tanker, the St James Park, was reportedly captured in the Gulf of Aden on Monday while on its way to Thailand from Spain.
Its 26 crew hail from Russia, Ukraine, Bulgaria, Romania, Poland, Georgia, India, Turkey and the Philippines.
The Navios Apollon, a Panamanian-flagged Greek cargo ship with 19 crew, was hijacked north of the Seychelles.
The 50,000-tonne carrier - which had been sailing from the US state of Florida to India - was boarded on Monday by 10 men in speedboats, according to the Greek coastguard.
Pirate attacks have been common off the Somali coast and international navies have been deployed to counter them.
'No contact'
The St James Park issued a distress call on Monday night saying it had been attacked by pirates, said the International Maritime Bureau.
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The tanker, which was carrying a cargo of chemicals used to make plastics, had changed course and was now heading towards the northern coast of Somalia, said Andrew Mwangura of the Seafarers Assistance Programme.
"It is expected to arrive there later this evening," Mr Mwangura told AFP news agency.
The ship's owner-operators told the BBC they were working for "the early and safe release of our crew members on board".
A spokesman for Zodiac Maritime Agencies said the company had had no contact with the St James Park or the pirates but was in contact with the European naval force in the area.
The waters around Somalia are among the most dangerous in the world.
Correspondents say the upsurge in piracy in the region is a consequence of the failure to find a solution to Somalia's continuing political disarray.
Pirates from Somalia have captured two ships in the Gulf of Aden on Monday.  The ships thought to be captured are the British owned St James Park chemical tanker, and the Panamanian Navios Apollon cargo ship.  This brings the total number of ships being held by the pirates to ten.  Also on Monday, pirates released a Singapore-owned cargo ship, the Kota Wajar, for US$4 million (€2.7m, £2.5m) ransom.
Gulf of Aden 
The pirate commander Mohamed Shakir told ''The Times'' “We have hijacked a ship with a British flag in the Gulf of Aden late yesterday, we have peacefully captured the ship and no shots were fired and there are no casualties.”  The ''St James Park's'' owner Zodiac Maritime Agencies released a statement saying that none of the crew had been injured and that they are working to get the crew members released.

Somalia's provisional government has vowed to fight piracy in their territory, although the pirates appear to outnumber the government.  The government receives aid from foreign nations and the European Union to help fight the piracy.  Large ransoms appear to be the motive for young Somalians and foreigners to hijack these ships.