Pirates have hijacked an oil tanker off the coast of Somalia, Somali officials and piracy experts said Tuesday, in the first hijacking of a large commercial vessel there since 2012.

The area where the hijacking occured is overseen by the U.S. Navy's 5th Fleet, which is based in Bahrain.

It was not immediately clear what the pirates' intentions are, but it may become one of the Trump administration's first international tests.

The Aris 13 on Monday reported being approached by two skiffs, John Steed with the organization Oceans Beyond Piracy said. The ship had been carrying fuel from Djibouti to Somalia's capital, Mogadishu, Steed said.

An official in the semiautonomous state of Puntland said over two dozen men boarded the ship off Somalia's northern coast. Another Puntland official said the ship was being moved toward the coast. Both officials spoke on condition of anonymity because they were not authorized to speak to the press.

A Britain-based spokeswoman for the European Union Naval Force operation off Somalia, Flt. Lt. Louise Tagg, confirmed that an incident involving an oil tanker had occurred and an investigation was underway.

An official said no ransom demand had been made.

"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," the official said. "The ship changed course quite soon after that report and is now anchored." The official spoke on condition of anonymity as no one was authorized to speak publicly about the incident.

It was not immediately clear who owned the ship or there it was flagged. Steed said it was UAE-owned and Sri Lankan-flagged, but the Middle East-based official said it was Greek-owned and Comoros-flagged with plans to re-flag it to Sri Lanka.

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.

But frustrations have been rising among local fishermen, including former pirates, at what they say are foreign fishermen illegally fishing in local waters.

Salad Nur, an elder in Alula, a coastal town in Puntland, told the AP by telephone 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."

Somali pirates usually hijack ships and crew for ransom. They don't normally kill hostages unless they come under attack, including during rescue attempts.

This would be the first commercial pirate attack off Somalia since 2012, Steed said.

A United Nations report seen by the AP in November said it had been almost three years since Somali pirates successfully hijacked a large commercial vessel, but they retain the capacity and intent to resume the attacks and lately have shifted to targeting smaller foreign fishing boats.

The EU force website currently lists no vessels or hostages held by pirates.

Concerns about piracy off Africa's coast have largely shifted to the Gulf of Guinea.

The Associated Press contributed to this report