MOSCOW - More than 300 sailors are being kept hostage by Somali pirates, who have dealt a severe blow to busy shipping routes off the country, a Russian maritime news website said Wednesday.

The editor of Sovfracht Maritime Bulletin said the pirates have seized some 50 vessels this year, including two yachts, and more than 25 Russian sailors, though some have been released.

'More than 300 sailors are being held captive, to be precise 287 plus an indefinite number of crew members from fishing boats of Yemen and Kenya,' Mikhail Voitenko said.

'The average ransom they demand is $1.8 million per vessel, and 60 days is the average amount of time spent in captivity.'

The editor said 15 vessels have been released for ransom in the last half a year, and the pirates are still holding 17 boats and two yachts.

The current captives include 11 Russians, one Estonian and one Georgian on board the ship CEC Future as well as two Russians, 17 Ukrainians and one Latvian on board the Faina. Eight Ukrainian nationals were also seized along with the Bosphorus Prodigy cargo ship.

Pirates have been increasingly active in the waters off Somalia, where more than 120 ships have been attacked this year. The navies of at least 16 countries are involved in anti-piracy operations off the coast of the East African nation.

The UN Security Council adopted a resolution in December authorizing countries and multinational organizations involved in tackling piracy to 'undertake all necessary measures in Somalia, including in its airspace' to prevent 'acts of piracy and armed robbery at sea'.