MOGADISHU (Reuters) - The largest ransom ever paid to Somali pirates was dropped on Sunday onto a Greek-flagged oil tanker with two million barrels of oil onboard, pirates and maritime officials said. An aircraft dropped a ransom believed to be between $5.5m (3.4 million pounds) and $7m for the release of the tanker which was hijacked near the Indian Ocean archipelago of the Seychelles, the officials said. The Maran Centaurus was seized on November 29 with nine Greeks, two Ukrainians, one Romanian and 16 Filipinos on board and the ransom dwarfs amounts paid previously for vessels held captive by Somali sea gangs. The tanker has yet to be freed as a dispute between rival pirate gangs over the spoils means the recipients are wary of returning to the coastal haven of Haradheere with their booty. Earlier on Sunday, pirates aboard the tanker and rivals in speedboats fired at each other in a tussle for control of the vessel before the ransom was due to be delivered. Pirates aboard a vessel anchored nearby and local elders onshore told Reuters helicopters from Western navies patrolling the waters off Somalia fired at the speedboats, clearing them from the area before the cash was dropped. The pirates in the speedboats had threatened to set the tanker ablaze if they did not receive a share of the spoils. 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. 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.