MOGADISHU (AFP) - Tension mounted Sunday between pirates holding a Saudi tanker and Muslim fighters threatening to attack them, with a week remaining for the ship's owners to meet a 25-million-dollar ransom demand. "If the pirates want peace, they had better release the tanker," Sheikh Ahmed, a spokesman for the Shebab group in the coastal region of Harardhere, told AFP by phone. The Sirius Star, a huge tanker carrying around 100 million dollars worth of crude oil and owned by Saudi Aramco, was hijacked in the space of 16 minutes by Somali pirates in the Indian Ocean on November 15. Pirates have since anchored it off their base in Harardhere, north of Mogadishu, and demanded the ransom be paid by November 30. The Shebab leaders have stressed that piracy is a capital offence under Islam and officially condemned the surge in acts of piracy in Somalia's waters, which has begun to disrupt international trade. A member of the pirate group holding the Sirius Star retorted that his own men were not afraid of the Shebab's threats. "We are the Shebab of the sea and we can't be scared by the Shebab of the land," Mohamed Said told AFP. "If anybody attempts to attack, that would be suicide." Said announced to AFP on Thursday that his group was demanding $25m to release the vessel, which is carrying the equivalent of almost a quarter of top producer Saudi Arabia's daily output. "I am not on the tanker at the moment because I am coordinating what is happening on the ground," he said. "There is a small Shebab vanguard on the ground but we also have a strong presence. Members of the pirate group told AFP on Saturday that talks were underway with Saudi Aramco's shipping arm and assured that the crew would not be harmed but added that no breakthrough had yet been achieved. Yet he warned against any plan by Saudi Arabia or foreign navies patrolling the region to free the ship by force. "I hope the owner of the tanker is wise enough and won't allow any military option because that would be disastrous for everybody. We are here to defend the tanker if attacked," Abdiyare Moalim said. The capture of the Sirius Star, the biggest ship ever hijacked, and its oil cargo, has sowed panic in the shipping world, with companies now re-routing deliveries via the Cape of Good Hope, around the tip of South Africa, adding substantial time and transit costs. With close to 100 attacks on ships in the Gulf of Aden and Indian Ocean this year, the pirates are threatening to choke one of the world's busiest maritime trade routes.