An EU Naval Force statement said the operation had finally made contact with the ship's master, who confirmed that armed men were aboard the Comoros-flagged tanker Aris 13.
Monday's hijacking was the first such seizure of a large commercial vessel off Somalia since 2012.
It came as a surprise to the global shipping industry as patrols by the navies of NATO countries, as well as China, India and Iran, had suppressed Somali pirate hijackings for several years.
A Somali pirate who said he was in touch with the armed men aboard the tanker said the amount of ransom to demand had not yet been decided.
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Bile Hussein told The Associated Press that the armed men have locked most of the crew in one room and cut off communication lines.
The Aris 13, manned by eight Sri Lankan sailors, was carrying fuel from Djibouti to Somalia's capital, Mogadishu, when it was approached by men in two skiffs, said John Steed, the director of Oceans Beyond Piracy. The EU statement said the ship's master issued a mayday alert.
An official in Somalia's semiautonomous state of Puntland said over two dozen men boarded the ship off the country's northern coast, an area known to be used by weapons smugglers and members of the al-Qaida-linked extremist group al-Shabab. The official spoke on condition of anonymity because they were not authorised to speak to journalists.
The EU Naval Force said it had passed the information from its contact with the ship's master to the oil tanker's owners and an investigation was underway.
Sri Lanka's Foreign Ministry said in a statement it was in touch with shipping agents and officials abroad for more information to help ensure the crew's "safety and welfare.
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