Somali pirates said they have freed the Panama-flagged merchant ship MV Orna after receiving a ransom of $400,000, but detained six of the crew to get more money for them.

The ship, a bulk cargo vessel with a deadweight of 27,915 tonnes, owned by the United Arab Emirates, was stopped by pirates firing rocket-propelled grenades about 400 nautical miles northeast of the Seychelles in December 2010.

Pirates said they released MV Orna held at Haradheere in central Somalia, but were still holding the captain, the engineer and four others of its 19 mostly Indian crew, Reuters reports.

"We now hold six men, including the captain and the chief engineer," pirate Mohamed told Reuters from Haradheere

The amount of the ransom for release of the ship could not immediately be verified independently.

"We have taken $400,000 and released MV Orna. The owners towed it with other ships," pirate Abdiweli told Reuters.

"We want ransom for them, but I cannot say the amount now."

Seaborne gangs are making tens of millions of dollars in ransoms, and despite successful efforts to quell attacks in the Gulf of Aden, international navies have struggled to contain piracy in the Indian Ocean owing to the vast distances involved.

However, naval ships have intensified their crackdown, including strikes on the pirates' coastal bases, and shipping firms are increasingly using armed guards and defensive measures on vessels, including barbed wire.

This has helped to reduce the number of incidents involving Somali pirates to 69 in the first half of 2012, compared with 163 in the same period last year, the International Maritime Bureau has said.