Somali pirates have released an Iranian-owned bulk carrier they hijacked 18 months ago, with the payment of a ransom ending one of the longest ship seizures, a maritime monitoring group said Monday.

The Maltese-flagged MV Sinin, which was seized on February 12, 2010 with its 21-member crew, was released late on Saturday, Ecoterra International said in a statement.

The news came after a ransom was paid, Ecoterra said without specifying the amount.

The ship was hijacked while on its way to Singapore from the United Arab Emirates.

Despite the presence of foreign naval vessels off the Somali coast, gunmen have continued to capture ships and rake in huge ransoms.

Currently at least 52 vessels and at least 573 hostages are still being held by the Somali pirates, according to Ecoterra.