Somali pirates are believed to be holding about 20 ships
Three ships held captive by Somali pirates have been moved after Islamist insurgents seized one of their bases.
The ships, reportedly with some 60 crew on board, have been sailed 110km (70 miles) up the coast from Haradhere to Hobyo, a pirate told the BBC.
The fate of some 300 other hostages, including a British couple, is unclear.
BBC East Africa correspondent Will Ross says complex ransom negotiations risk being undermined by a possible turf war after multi-million dollar payments.
Residents said several hundred insurgents from the Hizbul-Islam group took control of Haradhere over the weekend.
Hizbul-Islam wants to establish Sharia law in Somalia and put an end to the pirate trade in the town, the leader of the insurgents told the BBC.
But our correspondent says there are reports that this same militant group had earlier tried to strike a deal with the pirates to try to get a share of the profits.
Our correspondent says the shipping industry will watch the group's next moves very closely to get an idea of whether the vital shipping lanes look set to become safer or more dangerous.
Pirates have seized dozens of ships in the Indian Ocean in recent years, leading several nations to send warships to the area to protect commercial shipping.
- Admitted links with al-Qaeda
- Has foreign fighters in its ranks
- Well organised militarily and logistically
- Led by Hassan Dahir Aweys
- Aweys led al-Itihad al-Islamiya, put on US terror list in 2001
- Home-grown Islamist movement
Somalia has not had a functioning national government for nearly 20 years.
Our correspondent says the fate of more than 350 hostages currently being held by the pirates is precarious to say the least.
The pirates, divided into numerous different groups, are also holding about 20 ships in their various bases, such as Haradhere.
Pirate Ali Omar told the BBC's Mohammed Olad Hassan in Mogadishu by telephone that the ships had been moved to Hobyo.
The AFP news agency reports that these ships have a total of 60 crew, from countries including Myanmar and Kenya.
"There were three ships near the coast of Harardhere but this morning we cannot see them, they moved towards Hobyo," local fisherman Abdikafar Mohamed told AFP.
"I think the pirates are afraid of the Islamists and you cannot see them in town today, they fled, you cannot reach them on their cell phones as most of them headed towards Hobyo."
Hundreds of pirates could be seen leaving Haradhere in luxury cars hours before the insurgents moved in, local resident Suleyman Gadid told the BBC.
Hizbul-Islam and the hard-line Islamist group al-Shabab have a common agenda in fighting the UN-backed interim government and have previously shared control of the southern Somali port of Kismayo.
Somali analyst with the International Crisis Group Rashid Abdi told the BBC that he didn't think the Islamist groups would get involved with piracy, as they are trying to portray themselves as positive forces working for the national interests.
The two groups fell out in 2009 and al-Shabab, which has links to al-Qaeda, ousted Hizbul-Islam from the lucrative harbour-town.
Since losing Kismayo, the group has been keen to gain a foothold in Haradhere before al-Shabab moved in, says the BBC's Mohammed Olad Hassan in Mogadishu.