Somali pirates released a Greek supertanker and its crew of 28 on Monday after a rival pirate group attacked the pirates onboard in an unsuccessful attempt to steal the ransom, the spokesman for the European Union’s anti-piracy force said.

Cmdr. John Harbour says pirates left the Greek-flagged Maran Centaurus Monday morning. He said a group of rival pirates had attacked the ship just before the ransom was being delivered, prompting the pirates onboard the tanker to call for assistance from the anti-piracy force.

He said the EU naval force did not intervene but declined to give details on the actions of other warships in the area. The naval force is monitoring the ship as it leaves Somali waters, he said.

A Somali middleman, who helped negotiate for the release of the ship, says pirates collected $5.5 million Sunday afternoon and left the ship Monday morning. The figure could not be immediately confirmed.

The middleman, who spoke on condition of anonymity, said pirates in two speedboats attacked the ship Sunday just before the ransom was due to be delivered but after a brief shootout, two helicopters from a warship intervened. The middleman said the two helicopters did not fire at any of the pirates, but only hovered over them, successfully scaring off the attacking group.

The Maran Centaurus was hijacked November 29 about 1,300 kilometers off the Somali coast. It was carrying about 2 million barrels of crude oil from Saudi Arabia destined for the United States, estimated to be worth roughly $150 million at the time of the attack.

A Greek coast guard spokeswoman said the tanker had left Somalia escorted by a Greek frigate and was heading to the South African port of Durban. She said all crew members were in good health, and the ship was expected to reach Durban in a week. She spoke on condition of anonymity in line with Greek government regulations.

The ship, only the second oil tanker captured by Somali pirates, had nine Greeks, 16 Filipinos, two Ukrainians, and a Romanian aboard. Its seizure resurrected fears of an environmental or safety disaster first raised by the capture of the Saudi-owned Sirius Star. That hijacking was resolved in January last year with a $3 million ransom payment. It was carrying 2 million barrels of oil valued at about $100 million at the time.

The International Maritime Bureau said last week that sea attacks worldwide surged 39 percent to 406 cases, the highest in six years, with raids on vessels by Somali pirates accounting for more than half of the attacks.

It said that Somali pirates were responsible for 217 of the global attacks and had seized 47 vessels. This was nearly double the 111 attacks Somali pirates launched in 2008, of which 42 were successful hijackings.

The impoverished Horn of Africa nation has not had a functioning government for 19 years and the weak U.N.-backed administration is too busy fighting insurgency to arrest pirates. Pirates now hold about a dozen vessels hostage and more than 200 crew members.

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