Armed men attacked an overcrowded dinghy carrying refugees off Libya, leaving at least four dead and up to 25 missing, the , said.
The group said the Italian coastguard had sent a Sea Watch rescue ship and a diverted oil tanker to help a dinghy in distress in international waters, 14 nautical miles off Libya, early on Friday.
As the operation was unfolding, the crew of a speedboat labelled "Libyan Coast Guard" attacked the boat packed with about 150 refugees, beating them with sticks and causing at least four to fall into the water and drown, Reuters news agency reported.
"The violent intervention of the Libyan Coast Guard caused a situation of mass panic on board the rubber boat in distress," Sea Watch said in a statement.
"One tube of the rubber boat collapsed, causing the majority of the 150 people to slip into the water."
Sea Watch said four bodies had been recovered and 120 people were rescued. Search operations were still under way, Sea Watch spokesman Ruben Neugebauer told Reuters.
The men, who spoke only Arabic, beat some of the refugees with clubs, causing panic that caused most of them to fall into the sea, said Neugebauer.
The Italian coastguard confirmed that the vessel SeaWatch2 rescued 120 people before dawn on Friday, but gave no further details.
A spokesman for Libya's naval forces in Tripoli said he had not heard about about the incident.
According to the United Nations, at least 3,654 people have died trying to cross the Mediterranean this year.
Attempts at the dangerous crossing continue despite worsening weather as winter approaches, with more than 2,400 migrants and refugees rescued off Libya since Sunday.
The Italian interior ministry said on Tuesday more than 145,000 refugees and migrants landed in Italy this year, a figure similar to the previous two years.