Somali pirates said Thursday that they were freeing a Ukrainian ship carrying tanks and other heavy weapons after receiving a $3.2 million ransom. The U.S. Navy said it was watching the pirates leaving the ship.

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

A spokesman for the owners said that the pirates had received a ransom but it was far below their original demand of $20 million.

Mikhail Voitenko said the pirates were leaving the ship in small groups on boats carrying portions of the ransom. U.S. seamen were inspecting the departing boats to make sure they weren't taking weapons from the Faina's cargo, Voitenko said.

Cmdr. Jane Campbell, a spokeswoman for the 5th Fleet in Bahrain, said the Navy was not taking action against the pirates because it did not want members of other crews still in captivity to be harmed.

"Even when you release Faina, there are still 147 mariners held hostage by armed pirates," Campbell told The Associated Press on Thursday. "We're concerned for their well-being."

Pirate spokesman Sugule Ali told the AP by satellite phone that the pirates were leaving the ship slowly because the waters are "a bit turbulent."

"The whole thing is practically over and done with," Ali said from the central Somali coastal town of Harardhere, near where the MV Faina is anchored. "Our plan is to abandon the ship today (Thursday), by early evening at the latest."

Ali said his group was paid a ransom of $3.2 million, which he said was dropped by plane.

"We are not holding it (the ship) now anymore," Aden Abdi Omar, one of the pirates who left the ship told the AP from Harardhere. "But our men should disembark first for it to move to wherever it wants."

The MV Faina was loaded with 33 Soviet-designed battle tanks and crates of small arms. In the past, diplomats in the region have said that the cargo was destined for southern Sudan, something the autonomous region has denied.

Spokesman Alfred Mutua repeated the Kenyan government's claim to the cargo Thursday.

Nina Karpachova, Ukraine's top human rights official, said the ship will go to Mombasa, Kenya. If the crew is able to start the main engine, the ship will sail under its own power; otherwise it will be towed. The U.S. Navy will help provide security for the ship, she said.

Karpachova confirmed that the crew onboard the MV Faina is comprised of 17 Ukrainians, two Russians and a Latvian.

As soon as all the pirates have left the ship, Ukraine expects the U.S. Navy to send doctors onboard to provide first aid to the sailors.

"It's understandable that their health is poor and they are psychologically exhausted," she said.

Piracy has taken an increasing toll on international shipping in the key water link between the Mediterranean Sea and the Indian Ocean. Pirates made an estimated $30 million hijacking ships for ransom last year, seizing 42 vessels off Somalia's 1,900-mile (3,000-kilometer) coastline.

Analysts say although attempts to hijack ships remain steady at around 15 a month, the pirates are proving less successful. The pirates took two ships in December and three ships since the beginning of the year, compared with seven in November and five in October.

Graeme Gibbon Brooks, managing director of the British company Dryad Maritime Intelligence Service Ltd., said the drop was partly attributable to activity by nations that have sent ships to deter attacks and partly to unseasonably bad weather. Most of the 16 attempted hijackings in 2009 occurred in the first two weeks of January, when the weather was good.

But the pirates were showing a worrying new sophistication in their attacks, he said, jamming emergency frequencies with Arabic music or sending out false distress calls to lure warships away.

Somalia does not have a coast guard or navy because it has not had a functioning government since warlords overthrew dictator Mohamed Siad Barre in 1991. They then turned on each other, reducing Somalia to anarchy and chaos.