There are fears if the ransom is paid other super-tankers will be targeted
Somali Islamist insurgents have begun searching for the pirates who hijacked a giant Saudi-owned oil tanker last Saturday, reports say.
A spokesman for the al-Shabab group, Abdelghafar Musa, said hijacking a Muslim-owned ship was a major crime and they would pursue those responsible.
The pirates are thought to be trying to obtain a multi-million dollar ransom.
The ship, the Sirius Star, is believed to be anchored off the Somali port of Haradheere.
It has an international crew of 25 people and is carrying $100m (£67m) worth of crude oil.
'Show of force'
"We are really sorry to hear that the Saudi ship has been held in Somalia," Mr Musa told the Associated Press.
"We will fight them."
Reports said Islamist fighters had descended on Haradheere in an apparent show of force, saying they were looking for the pirates.
"The Islamists arrived searching for the pirates and the whereabouts of the Saudi ship," an unnamed elder in the port told Reuters news agency.
"I saw four cars full of Islamists driving in the town from corner to corner. The Islamists say they will attack the pirates for hijacking a Muslim ship."
Another report suggested local militia and Shebab fighters had arrived in Harardhere in a move to position themselves for a share of any spoils.
"There are many militiamen who have arrived in the town and they want to get a share from the pirates if the ransom is paid," Ahmed Abdullahi, a local elder, was quoted by AFP news agency as saying.
Meanwhile, Kenya reported on Friday that Somali pirates had been paid more than $150m (£101m) in ransoms in the past 12 months.