On Thursday Somali pirates freed a Ukrainian ship carrying tanks and other heavy weapons after receiving a $3.2 million ransom.

The U.S. Navy watched the pirates go but was however not able to act as the pirates still hold almost 150 people hostage from other crews and didn't want to put any of their lives at risk

The seizure of the MV Faina was one of the most brazen in a surge of pirate attacks on shipping off the Somali coast. Vessels from the U.S. Navy's 5th Fleet quickly surrounded it after it was seized in September, to make sure the cargo did not get into the hands of Somali insurgent groups believed to have links to al-Qaida.

U.S. seamen were inspecting the pirates' departing boats to make sure they weren't taking weapons from the Faina's cargo, Mikhail Voitenko, a spokesman for the ship's owners, said Thursday, The Associated Press reported.

The ransom was dropped off by plane.

The amount of $3.2 million was far below the pirates' original demand of $20 million.

Piracy has taken an increasing toll on international shipping, especially in the Gulf of Aden, one of the world's busiest shipping lanes. Pirates made an estimated $30m hijacking ships for ransom last year, seizing more than 40 vessels off Somalia's 3 000km coastline.

Somali waters are now patrolled by more than a dozen warships from countries including Britain, France, Germany, Iran and the United States.