(CNN) -- U.S. military officials expressed concern Monday that Somali militants believed to be potentially affiliated with al Qaeda may become involved in the hijacking of a supertanker off eastern Africa.
Five armored vehicles loaded with al-Shabab fighters arrived in the central Somali coastal town of Harardhere this weekend, near where pirates are holding the massive Saudi Arabian oil tanker Sirius Star, said Ahmed Mohamed, a local resident.
The fighters told residents they would battle the pirates because the tanker, loaded with 2 million barrels of oil worth about $100 million, is owned by a Muslim country and should not have been taken, Mohamed said.
The al-Shabab militia is an offshoot of an Islamic party that ruled much of Somalia in the second half of 2006 and aims to impose Islamic sharia law in Somalia. It was forced from power by Ethiopian troops but continues to fight for control.
The U.S. State Department included the group on its register of foreign terrorist organizations in March of this year -- and declared it had links with al Qaeda.
"Al-Shabab is a violent and brutal extremist group with a number of individuals affiliated with al Qaeda," the department said on its Web site. "Many of its senior leaders are believed to have trained and fought with al Qaeda in Afghanistan."
"We're certainly concerned about the interest of Islamic extremists in piracy," said a U.S. military official monitoring developments Monday.
"We need to learn more about what al-Shabab is up to. Whatever influence al-Shabab is interested in, is troublesome" he said.
Another resident of Harardhere, Hassan Nor, suggested that al-Shabab's motive was to share in the multi-million dollar ransom the pirates have demanded from the supertanker's owner.
Until now the U.S. military has emphasized that piracy issue is a criminal matter that must be dealt with by the international community.
U.S. officials have long said that if they could determine a connection to terrorism, it could potentially require more military action in addition to the warships patrolling the region, although they have not been specific about what they might do.