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."