The Georgian captain of a cargo ship that was released by Somali pirates said Tuesday his Georgian-Turkish crew had suffered through "infernal torment" during its 18-month captivity.

"We have gone through an infernal torment, starvation, thirst. All this time we have been on a leash, like animals," captain Memed Zakaradze told Georgia's Rustavi 2 television by telephone from an unspecified location.

"But we did not lose hope for even a second," he said.

Armed raiders boarded the Malta-flagged cargo ship in the piracy-plagued area off the Gulf of Aden in September 2010 and seized the crew of 15 Georgians and three Turks.

The ship had been on its final voyage to India to be scrapped and its Greek owner had gone into liquidation, leaving the sailors effectively stranded while the pirates demanded $9 million (7.1 million euros) to release the ship.

Georgian President Mikheil Saakashvili said the sailors' release Sunday was secured through confidential negotiations whose details he could not disclose at this time.

"Georgia will always fight for its citizens," Saakashvili said on Tuesday in televised remarks.

While refusing to provide any details, Georgian officials have denied suggestions that a ransom was paid.

"No ransom whatsoever was paid to the pirates," Maritime Transport Agency spokeswoman Ana Gomarteli told AFP.

She said all the sailors were currently heading to "a safe port in Africa under armed guard" and would return to Georgia by Monday.

"After their return to Georgia we will be able to speak in details about how their liberation was achieved," she added.

Piracy off the coast of Somalia, on a crucial maritime route leading to the Suez Canal, surged in 2007 and reached record levels in 2010 according to a report last year by the International Maritime Bureau.

According to Ecoterra International, an environmental and human rights organisation that monitors regional maritime activity, pirates currently hold at least 43 ships and more than 400 seamen.