"If they do not get the money they are demanding, we call on them to either burn down the ship and its arms or sink it," Sheikh Mukhtar Robow, a spokesman for the Shabab movement, said.
Robow said his group, which is gradually gaining ground over government troops in southern Somalia, was not linked to the pirates who seized the Belize-flagged freighter last week as it headed for Mombasa in Kenya.
"We have no contacts and links with the pirates and they are in the waters for their own interests. It is a crime to take commercial ships but hijacking vessels that carry arms for the enemy of Allah is a different matter," Robow said.
|US warships are blockading the hijacked vessel MV Faina off the Somalian coast [AFP]
His group nearly stamped out piracy when it controlled southern Somalia last year.
Robow claimed that the 33 Soviet-era T72 battle tanks and other military hardware on the MV Faina belonged to Ethiopian forces, who provide crucial support to the Somali government in the capital Mogadishu.
"We believe that the military shipment belonged to Ethiopia and was headed to Mogadishu sea port, where it would have been unloaded with the intention of destroying Somalia, but that never happened," he said.
Earlier in the week, the pirates said the arms were headed for Sudan.
The Ukrainian owners of the freighter and Kenyan government said the tanks were destined for Kenya.
The US navy has vowed to prevent the pirates from offloading the arms. The defence department said it expected a Russian warship to arrive within days, but said it had laid the emphasis on ensuring a "peaceful resolution".
A Russian naval spokesman also played down talk of confrontation, denying in a statement on Wednesday evening that the vessel had been sent to fight the pirates, who are holding Russia seaman among the crew of the MV Faina.
"Such declarations are of a provocative character, they can prejudice the talks talks aimed at freeing the crew of the cargo ship," Igor Dygalo, a Russian navy spokesman, said.
The patrol boat Intrepid despatched from the Baltic fleet to the Indian Ocean was there to protect Russian shipping there, he said.
Abdullahi Yusuf Ahmed, the Somali president, earlier urged Somalis and the international community to combat rising piracy off the lawless nation's waters.
"They (pirates) are imposing an embargo on the Somali people and the international community because they are blocking movement between the Mediterranean and the Indian Ocean, which affects not only Somalia but the whole world," Yusuf said in Mogadishu.
"I call on the Somali people to fight against the pirates. I also call on the international community to act quickly on what is happening in Somali waters as well as on shore."
Meanwhile, a meeting of EU defence ministers in Deauville, northern France, agreed to launch an anti-piracy security operation off the coast of Somalia, Herve Morin, the French defence minister, said Wednesday.
"There is very broad European willingness," he said.
"Many countries want to take part. Ten have clearly given their accord to take part in such a mission."
They include Belgium, Cyprus, France, Germany, Lithuania, the Netherlands, Spain, Sweden and possibly Britain, he said.