By Mohamed Ahmed and Abdi Guled

MOGADISHU (Reuters) - Rival Somali pirate gangs fired shots at each other on Sunday in a dispute over how to split any ransom for a hijacked Greek-flagged oil tanker with two million barrels of crude oil aboard.

Pirates from the semi-autonomous northern region of Puntland, who seized the Maran Centaurus in November, say they do not want to a share the spoils with pirate gangs in the pirate haven of Haradheere, where the vessel is moored.

But the tanker and its 28 hostages are now under the control of gunmen from Haradheere. The pirates from Puntland, aboard speedboats nearby, are threatening to set fire to the tanker if they miss out on any payment.

"We have risked our lives in hijacking the ship. These Haradheere men cannot deprive us of our rights," a pirate called Aden told Reuters. "If need be, we shall start a fire as soon as the ransom is about to arrive."

The United Nation's shipping agency warned in December that if the tanker had an accident due to bad weather off the coast of Somalia it would create an "environmental catastrophe".

The ship's Greek management company says there are nine Greeks, two Ukrainians, one Romanian and 16 Filipinos on board.

Many residents fled the centre of the normally peaceful Haradheere on Sunday morning fearing violence could engulf the coastal town north of the capital -- or allow hardline Islamist insurgents to take advantage of the situation and move in.

"Our town was calm and booming but now we fear violence," local elder Abduallahi Ali told Reuters by telephone. "The pirates are well-armed and if they exchange fire it will affect the whole area."

ATTACKS ON THE RISE
Somali pirates had a bumper year in 2009. Worldwide piracy attacks surged nearly 40 percent, with Somali pirates accounting for more than half of the 406 reported incidents, according to the International Maritime Bureau (IMB).

Typically, the pirates hold the ships and crew hostage until they are paid ransoms and free the vessels. With ransom payments running into millions of dollars, the stakes are high for the gunmen in their poor, anarchic Horn of Africa nation.

At the end of last year, Somali pirates were holding at least 12 vessels for ransom with 263 crew members of various nationalities as hostages, the IMB said last week.

Ransoms are usually divided between the hijackers -- with bigger shares going to those who first boarded the vessel -- people who have invested in the pirate ventures, those who guard moored ships and local communities onshore.

There is even a small pirate "stock exchange" in Haradheere where a Somalis can contribute money or weapons to the sea gangs in return for a dividend when ransoms are paid. In Puntland, by contrast, the authorities have pledged to crack down on piracy.

"The argument started after men from Puntland who hijacked the ship refused a ransom share for Haradheere pirates. Now hundreds of well-armed and angry pirates have gone onto the ship," pirate Hassan told Reuters.

"Puntland pirates cannot really challenge the Haradheere pirates because of the geographical position," he said. "Though the situation is tense as Puntland pirates may open fire to stop the ransom being taken by Haradheere pirates."