Divers, meanwhile, pulled out a woman's body from the capsized Costa Concordia on Sunday, raising to 13 the number of people dead in the Jan. 13 accident.
Officials told reporters that the victim was wearing a life vest and was found in the rear of a submerged portion of the ship.
Earlier, Italian authorities raised the possibility that the real number of the missing was unknown because some unregistered passengers might have been aboard. As of Sunday, 19 people are listed as missing, but that number could be higher.
Meanwhile, rescuers resumed searching the above-water section of the Concordia but choppy seas kept divers from exploring the submerged part, where officials say there could be bodies.
"There could have been X persons who we don't know about who were inside, who were clandestine" passengers aboard the ship, Franco Gabrielli, the national civil protection official in charge of the rescue effort, told reporters.
The ship rammed a reef on the island of Giglio and sliced open its hull and turned over on its side with 4,200 people aboard.
Gabrielli said relatives of a Hungarian woman told Italian authorities that she had telephoned them from aboard the ship and that they haven't heard from her since the accident.
He said it was possible that a woman's body pulled from the wreckage by divers on Saturday might be that of the unregistered passenger. However, the identity of that body and of three male bodies, all badly decomposed after days in the water, have yet to be established.
Gabrielli said they have identified the other eight bodies: four French, an Italian, a Hungarian, a German and a Spanish national.
So far, the Concordia's fuel tanks are holding up. Special crews are waiting for the end of rescue efforts to begin extracting the ship's nearly half a million gallons of heavy fuel.
The Associated Press contributed to this report.