NAIROBI, Jan 13 (Reuters) - Somali pirates have freed a Korean-owned bulk carrier they hijacked three months ago, a regional maritime official said on Tuesday.

The Panama-flagged, Japanese-operated African Sanderling was seized with its 21 Filipino crew on Oct. 15, all of whom were in good health. It was not clear if any ransom had been paid. "The vessel was released very late on Sunday night, but we just got word of it now," Andrew Mwangura of the Mombasa-based East African Seafarers' Assistance Programme told Reuters.

His organisation has been tracking a sharp rise in piracy off Somalia and in the busy Gulf of Aden. Last year, gunmen from the anarchic Horn of Africa nation hijacked dozens of ships and made tens of millions of dollars in ransom payments.

The attacks have raised insurance costs, prompted some owners to sent their ships around South Africa instead of via the Suez Canal and triggered an unprecedented deployment by foreign navies.

On Friday the pirates released a Saudi Arabian supertanker, the Sirius Star, after a $3 million ransom was parachuted onto its deck. The vessel was carrying crude oil worth $100 million.

The gunmen are still holding a Ukrainian ship loaded with 33 T-72 tanks, the MV Faina, which was hijacked in September.

The pirates say they hope it will be freed soon and that they had cut their ransom demand to $5 million from $20 million.

In a Reuters interview on Saturday, Somalia's interim President Sheikh Aden Madobe said ransom payments only encouraged piracy and that the problem must be tackled on land. (Reporting by Daniel Wallis; Editing by Matthew Jones)

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