"Her crew usually comprises 17 sailors and, based on outdated crew lists, it could be assumed that they are holding Romanian and Libyan nationalities," Ecoterra said.
The group said local reports suggested the hijackers were from Somalia's semi-autonomous northern region of Puntland.
"It is assumed the vessel is now commandeered to one of the Puntland pirate lairs," it said in a statement.
A EU counter-piracy force said a coalition ship, the USS Porter, and a helicopter from the USS Farragut confirmed the hijacking, but had no details on the nationalities of the crew.
"The MV Rim ... has now altered course and is heading towards the Somali Basin. Coalition forces will now monitor the situation," Navfor said in a statement.
The number of piracy attacks worldwide increased almost 40 per cent last year, with gunmen from the failed Horn of Africa state accounting for more than half the 406 reported incidents, according to the International Maritime Bureau.
Typically, the heavily armed Somali pirates hold captured ships and their crews hostage until ransoms are paid.
Earlier this week, maritime officials and pirate sources said a ransom was paid for the release of a Marshall Islands-flagged cargo ship, the MV Filitsa.
As their ransoms demands increase, rivalry between pirate gangs has grown.
A dispute in January over the biggest ever payoff, for a Greek tanker carrying 2 million barrels of oil, triggered deadly gunbattles at sea and then back on land.
The International Chamber of Shipping, which represents 75 per cent of the global seaborne industry, said last month that it felt deepening frustration at the international community's "impotence" in combating increasing piracy in the Indian Ocean.