An EU Naval Force statement late Tuesday said the operation finally made contact with the ship's master in late afternoon. He 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 on the crucial global trade route since 2012.
A Somali pirate who said he is in touch with the armed men says the amount of ransom has not been decided.
The reported seizure of the Aris 13 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.
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.
An official in the semiautonomous state of Puntland said over two dozen men boarded the ship off Somalia's northern coast, an area known to be used by weapons smugglers and members of the Al Qaeda-linked extremist group
The ship was anchored Tuesday off the town of Alula, said Salad Nur, a local elder. "The ship is on the coast now and more armed men boarded the ship," he told the Associated Press by phone.
"The vessel's captain reported to the company they were approached by two skiffs and that one of them they could see armed personnel on board," an official based in the Middle East said. "The ship changed course quite soon after that report and is now anchored." The official was also not authorized to comment publicly about the incident and spoke on condition of anonymity.
A Britain-based spokeswoman for the European Union Naval Force operation off Somalia, Lt. Louise Tagg, said an investigation was underway. The U.S. Navy's 5th Fleet, which is based in Bahrain and oversees anti-piracy efforts in the region, did not immediately respond to a request for comment.
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."
A U.N. shipping database shows the Aris 13 is owned by a company called Armi Shipping SA, whose address is listed in care of Aurora Ship Management FZE, a company based in Fujairah in the United Arab Emirates. Calls and emails to Aurora went unanswered.
Australian government records from 2014 list the ship's owner as Flair Shipping Trading FZE in the UAE. Argyrios Karagiannis, the managing director of Flair Shipping, declined to comment.
An address listed for Flair Shipping in Dubai's high-rise neighborhood of Jumeirah Lake Towers was for a company called Flair Oil Trading DMCC. A woman who answered the door Tuesday told an AP reporter the firm wasn't connected to the ship and directed him to another office.
When no one answered the door at that office, the AP reporter returned to find Karagiannis entering the office of Flair Oil Trading DMCC.
"We will not be releasing any information," Karagiannis said before shutting the door.
The incident involving the Aris 13 represents the first commercial pirate attack off Somalia since 2012, Steed said.
"The pirates never went away, they were just doing other forms of crime and if any of the measures reduce (which they have, or ships take risks) the pirates are poised to exploit the weakness," he said in an email.
Somali pirates usually hijack ships and crew for ransom. They don't normally kill hostages unless they come under attack.
Piracy off Somalia's coast was once a serious threat to the global shipping industry. It has lessened in recent years after an international effort to patrol near the country, whose weak central government has been trying to assert itself after a quarter-century of conflict. In that time, concerns about piracy off Africa's coast have largely shifted to the Gulf of Guinea.
But frustrations have been rising among local fishermen, including former pirates, at what they say are foreign fishermen illegally fishing in local waters.
Nur, the local elder, told the AP that young fishermen including former pirates have hijacked the ship.
"They have been sailing through the ocean in search for a foreign ship to hijack since yesterday morning and found this ship and boarded it," he said. "Foreign fishermen destroyed their livelihoods and deprived them of proper fishing."
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2:25 p.m.: This article was updated with news of the pirates demanding ransom.