Most of the sailors being held captive have been locked in a room, with lines of communication cut off to frustrate any rescue attempts.
The tanker has now been anchored near the Somali town of Alula, and a local elder says "more armed men have boarded the ship".
Weapon smugglers are known to operate in Alula, as well as members of al Shabaab, an extremist group with links to al Qaeda.
The hijacking began on Monday when the captain of Aris 13 sent a mayday alert as two skiffs closed in on the tanker in the Gulf of Aden.
Although the EU Naval Force made several attempts to make contact with the captain, they only received confirmation last night that armed pirates had stormed the ship and were demanding an unspecified ransom.
This is the first hijacking in the region for five years, and maritime experts have accused ship owners of becoming complacent after a long period of calm.
In October, the United Nations warned that the situation remained fragile, and Somali pirates still "possess the intent and capability to resume attacks".
Local aid groups said pirates in the region were still engaged in criminal activity, and had been waiting to "exploit the weakness" of ships that took unnecessary risks.
According to Oceans Against Piracy, Aris 13 was on a high-risk route, which is often used to save time and money.
"This attack reinforces the need for vessels to follow shipping industry best management practices within the specified high risk area," the organisation said.
The eight crew on board are all believed to be Sri Lankan nationals, and the country's foreign ministry said it is working to help ensure their "safety and welfare".
Pirates do not normally kill hostages unless they come under attack.
The local elder said young men in the area had turned to piracy because their livelihoods had been "destroyed" by foreign fishermen.
"They have been sailing through the ocean in search for a foreign ship … and found this ship and boarded it," Salad Nur told the Associated Press.
The ship's owner has not commented on the hijacking.