Reuters said data from its systems showed the ship made a sharp turn just after it passed the Horn of Africa on its voyage from Djibouti to Mogadishu.
The news agency said it was told by John Steed of the organisation "Oceans Beyond Piracy," the Aris 13 sent a distress call on Monday, turned off its tracking system, and altered course for the Somali port town of Alula, American Shipper reported.
"The ship reported it was being followed by two skiffs. Then it disappeared," Mr Steed told Reuters.
He said aircraft from regional naval force EU Navfor were flying overhead to track the ship.
The International Maritime Bureau's (IMB) Piracy Reporting Centre (PRC) said that in 2016, there were just two piracy incidents off Somalia. That's a big change from 2011 when Somali pirates launched 237 attacks and held hundreds of hostages.