The agency is trying to confirm what happened to the passengers when the vessel broke apart at sea shortly after leaving a port near Libya’s capital Tripoli on Friday, spokeswoman Laura Boldrini said.
There was no information on how many people might have died, and the uprising in Libya makes any official accounting unlikely.
Witnesses who departed on another boat shortly after reported seeing the ship broken apart and bodies floating in the sea, Boldrini said. The second boat with witnesses aboard arrived later in Italy, she said.
The number of migrants fleeing North Africa’s shores have increased since the region has been engulfed in a series of uprisings.
At least three other boats that departed Libya in late March have disappeared, with hundreds feared dead, Boldrini said.
She said that the UNHCR advised the Italian Coast Guard at the time that boats carrying 120 and 360 migrants respectively had departed Libya in late March but never arrived in Italy.
The Italian Coast guard later told the UNHCR that they were not able to come up with any information, she said.
The fate of those migrants is also not clear, but their relatives fear they are dead since they have lost contact with them. In both cases, the ships had satellite communications before going missing.
Boldrini said the incidence of deaths and disappearances among seafaring migrants fleeing unrest and repressive regimes in Africa is increasing as smugglers begin to use bigger boats that they are not capable of controlling.
Another aid group, the International Organization for Migration, said it is impossible to know how many people have drowned while trying to reach Europe.
“There’s been no way of charting for sure how many boats have left, how many people never made it. Some of them we will never know about,” said IOM spokeswoman Jemini Pandya.
In another case, relayed by an Eritrean priest in Rome to Boldrini, a boat with more than 70 people on board ran into trouble in the seas, she said. Only a handful survived, making their way back to Libya, according to the priest.
London’s Guardian newspaper reported on Monday that 61 African migrants died of hunger and thirst on that ship after being ignored by a NATO warship and helicopter in March around the time the alliance was readying for military strikes against the Gadhafi regime.
NATO denied the accusation, saying in a statement that the only vessel operating on the specified dates — March 29 or 30 — in that area was the Italian ship Garibaldi, and that it was 100 nautical miles out to sea, while the ship is believed to have floundered closer to shore.
“Therefore, any claims that a NATO aircraft carrier spotted and then ignored the vessel in distress are wrong,” the military alliance said.
UNHCR’s Boldrini, meanwhile, called for an improvement in communication between coast guards, military and commercial ships.
“We need take heed of a situation that is very much evolving. We have to cooperate much more closely,” she said.
In some instances, she said, ships in the heavily trafficked Mediterranean might see the boats but not intervene if there isn’t an obvious mechanical problem or emergency.
“Rescue should be automatic, without waiting for the boat to break apart or the engine to stop running,” Boldrini said.