Gunmen hijacked the ship last Wednesday, said Andrew Mwangura, program co-ordinator of the East Africa Seafarers Assistance Program. He did not know the nationalities of the crew on board or their number.
Two other ships were attacked off the Somali coast Saturday, with pirates firing on one of the boats, he said.
On Sunday, pirates in two speedboats attempted to seize a ship carrying cargo for the World Food Program - the third attack on a WFP ship this year, said Peter Smerdon, a spokesman for the UN program.
Not counting the attacks of the last week, Somalia has reported 26 hijackings this year - compared with eight in the same period last year, the International Maritime Bureau said.
Some hijackings have turned deadly: Pirates killed a crew member after seizing a Taiwan-flagged fishing vessel in May off the northeastern coast of Somalia.
The Almarjan, seized last Wednesday, was flying under a Comoros Island flag and was operated by Dubai-based Biyat International, Mwangura said. The incident took several days to confirm, he said Monday.
Mwangura said the rise in hijackings could be linked to the overthrow of an Islamic group that had cracked down on piracy after seizing control of the Somali capital and much of southern Somalia last year.
The Islamic group was driven out in an Ethiopian-led, U.S.-backed invasion last year that put another faction in power. The Islamists have since engaged in an increasingly violent Iraqi-style insurgency.
The country's coasts now are virtually unpoliced, and the shaky, UN-backed transitional government comes under daily attack by insurgents.
Somalia has not had a functioning national government since dictator Mohamed Siad Barre was overthrown in 1991.
Source: October 22,2001