Aid group Oceans Beyond Piracy said in a statement late on Tuesday the ship was carrying gas and fuel and was not registered with the Maritime Security Center for the Horn of Africa, which registers and tracks vessels in the region.
It came after a gunfight between naval forces from the semi-autonomous state of Puntland and the gang, followed by negotiations involving local elders.
Four people were wounded in the exchange of fire on Thursday, the BBC has learned. The bunkering tanker - the first commercial vessel hijacked off the Somali coast since 2012 - has a crew of eight Sri Lankan sailors who it is believed are still being held captive.
Coastal Somalis, including pirates who quit as worldwide patrols increased and became fisherman, have complained of growing harassment by illegal fishermen and attacks by large foreign trawlers.
Pirates on board a hijacked oil tanker anchored off the Somali coast have fired shots on maritime forces.
It was headed to port in the Somali capital Mogadishu.
Puntland authorities said earlier that they will send forces to the oil tanker kidnapped by pirates at the coast under its administration. The official, who spoke on condition of anonymity because he was not authorized to speak to the media, said pirates on the ship were continuing to receive reinforcements while regional forces mobilizing nearby.More news: Jose Mourinho predicts Manchester United defeat against Middlesbrough
He said the Puntland naval forces were dispatched to the area not to free the ship by force but to cut off any supplies to the pirates.
Australian government records from 2014 show the ship in Monday's incident was owned by Flair Shipping Trading FZE in the United Arab Emirates and linked to UAE-based ship management firm Aurora Ship Management FZE. However, they are often harassed by illegal fisherman and large foreign trawlers off the country's north coast.
The Sri Lankan foreign ministry has confirmed that eight of its nationals are on board the vessel.
Piracy off the coast of Somalia, usually for ransom, has reduced significantly in recent years, in part because of extensive global military patrols as well as support for local fishing communities.
In 2015, Somali officials warned that piracy could return unless the worldwide community helped create jobs and security ashore, as well as combating illegal fishing at sea.
Many Somalis, including former pirates, depend on fishing to make a living.