The Shebad (which means youth) armed group has issued an ultimatum to the pirates holding the Sirius Star, free the ship or be attacked.
The Shebab (meaning "youth") armed group controls much of southern and central Somalia. Sheikh Ahmed, a spokesman for the Shebab group in the coastal region of Harardhere said "If the pirates want peace, they had better release the tanker."
The Shebab
has positioned fighters in and around Harardhere in recent days.
The Sirius Star, was hijacked by Somali pirates in the Indian Ocean on November 15. The ship is anchored off their base in Harardhere and the pirates have demanded a ransom of $25m be paid by November 30.
Islamist leaders state piracy is a capital offence in Islam and officially condemned the surge in acts of piracy in Somalia's waters.
The pirates have said they are not afraid of the Shebab's threats.
Mohamed Said said: "We are the Shebab of the sea and we can't be scared by the Shebab of the land. If anybody attempts to attack, that would be suicide."
Said said: "I am not on the tanker at the moment because I am co-ordinating what is happening on the ground. There is a small Shebab vanguard on the ground but we also have a strong presence."
He added: "Every Somali has great respect for the holy kingdom of Saudi Arabia. We have nothing against them but unfortunately what happened was just business for us and I hope the Saudis will understand."
The idea of attacking the ship has been played down on the grounds that it would be too dangerous and would probably result in the deaths of some or all of the hostage crew.