Armed men are demanding a ransom for the release of an oil tanker they have seized off the coast of Somalia and the crew is being held captive, the European Union anti-piracy operation in the region announced late on Tuesday.

An EU naval force statement said the operation had finally made contact with the ship’s master, who confirmed that armed men were onboard 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.

However, the United Nations warned in October that the situation was fragile and that Somali pirates “possess the intent and capability to resume attacks”. One expert said some in the region had let down their guard as the situation calmed. Nato ended its anti-piracy mission off Somalia in December.

A Somali pirate who said he was in touch with the armed men onboard the tanker said the amount of ransom to demand had not yet been decided.

Bile Hussein told the Associated Press that the armed men have locked most of crew in one room and cut off communication lines. “Their main concern now is a possible rescue attempt, so that’s why all communications were cut off in the afternoon,” he said.

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 of Oceans Beyond Piracy. The EU statement says the ship’s master issued a mayday alert.

A Somali fisherman casts his nets at dusk on the beach in the former pirate village of Eyl, in Somalia’s semiautonomous state of Puntland . Photograph: Ben Curtis/AP

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-Shabaab. The official spoke on condition of anonymity because they were not authorised to speak to journalists.

The ship was anchored on 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 AP by phone.

An official based in the Middle East with knowledge of the incident told the AP that the vessel’s captain reported to the company it had been approached by two skiffs and that one had armed personnel on board. “The ship changed course quite soon after that report and is now anchored.” The official also spoke on condition of anonymity.

The EU naval force said it had passed information to the ship’s owners and an investigation was under way.

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 UN shipping database shows the Aris 13 is owned by a company called Armi Shipping SA, whose address is listed as 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 neighbourhood of Jumeirah Lake Towers was for a company called Flair Oil Trading DMCC. A woman who answered the door on 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 of a 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.”