Andrew Mwangura, director of the East African Seafarers Assistance Programme based at the Kenyan port of Mombasa, said the boats were taken off Puntland, a semi-autonomous province in the northeast of the Horn of Africa nation.
"They were taken by clan gunmen," Mwangura said.
Although local Somali media house Shabelle said the vessels belonged to Finnish firms, Mwangura said neither the flags of the vessels, nor the number and nationalities of the crews, were yet known.
Piracy has been rife off Somalia since the country descended into chaos after warlords toppled military dictator Mohamed Siad Barre in 1991. Many claim to be "coastguards" protecting their waters against illegal fishing and dumping of toxic waste.
The rise of Islamists, who ran most of south Somalia for the first half of 2006, had a strong deterrent effect on piracy. But attacks have been increasing since their fall in a brief war at the end of last year.
"Since the Islamists were kicked out, the number of hijackings has risen," Mwangura said.
Between February and April, there have been at least three vessels hijacked and four other attacks in which the vessels managed to escape or were rescued, he said.
Somalia's coastline is viewed by sailors as one of the most dangerous in the world.
- Reuters - Thomson Reuters Foundation
- For more humanitarian news and analysis, please visit https://www.trust.org/alertnet