Initial reports said "several Libyan soldiers" boarded the vessel, located about 30 miles off the Libyan coast, in the early hours of Friday morning, although a spokesman for the Sicilian fishing association said it was "probably an act of piracy".
The vessel, which had seven fishermen on board, was reportedly then followed by a tugboat and taken to the Libyan port of Misrata.
"The state of alarm has been at the maximum level since the Italian embassy (in Libya) was closed," Giovanni Tumbiolo, the president of the Sicilian unit of Cosvap, the fishing association, was earlier quoted by Ansa as saying, adding that he had contacted the Libyan agriculture and fisheries minister.
"We are worried but at the same time hopeful, because the Libyan people have always been close to the Sicilians."
Three of the fishermen on board were from Sicily and four were Tunisian.
Francesco Mezzapelle, a spokesman for the association, told Reuters that the seizure was “probably an act of piracy”, adding that Italy’s authorities were alerted by another Italian fishing boat.
The Italian foreign ministry is investigating the matter.
The chaos in Libya has seen thousands of migrants flee to Italy and also sparked fears of a new stronghold for jihadists to launch attacks on Europe.
Up to 400 people died trying to make the crossing on Sunday after their boat capsized off Libya’s coast.