Up to 25 people were missing and feared drowned Friday after men on a Libyan coastguard speedboat attacked a packed migrant dinghy during a rescue operation off the north African state.
German NGO Sea-Watch, which is taking part in the multinational search and rescue operation in the Mediterranean, said the tragedy happened after its boat Sea-Watch 2 and a passing oil tanker were sent to help the distressed dinghy in the early hours.
As the rescue operation proceeded just beyond Libyan territorial waters north of the port of Sabrata, a speedboat bearing the Libyan coastguard insignia arrived and tried to steal the dinghy's outboard engine, spokesman Ruben Neugebauer told AFP.
The men, who spoke Arabic, beat some of the migrants with sticks and some clambered onto the dinghy, causing panic which resulted in one side of the boat deflating and most of the passengers ending up in the sea.
After the assailants left, Sea-Watch said it rescued 120 people and recovered four corpses from the water.
Other bodies were seen floating but could not be recovered and it was estimated that between 15 and 25 of the people who had been on the board were unaccounted for.
Sea-Watch said in a statement that its two speedboats had been "hassled in an aggressive way" during the attack, "preventing our crew from providing life vests and medical aid to the people in need."
"All of these deaths could have been avoided but for this intervention," Neugebauer added.
- Over 3,500 deaths -
The spokesman said the NGO had no way of knowing if the attackers had any contact with the Libyan coastguard or had simply hijacked one of their boats.
But he said the incident highlighted the dangers inherent in European plans to train and equip the Libyan coastguard to be able to restrict the flow of migrant boats from the conflict-torn country towards Italy.
"It is hard to know who is doing what in Libya," he said. "It shows once again it is not a good idea to build a whole European policy based on these guys. You never know whose hands the equipment will end up in."
The EU training initiative is due to begin at the end of this month following vetting of around 80 Libyan coastguard staff who are due to take part.
A Libyan coastguard spokesman told AFP Friday they had not been aware of the incident involving Sea-Watch, which will send shockwaves through the humanitarian groups helping to save lives alongside navy and coastguard ships from a number of European countries.
"If confirmed it is very alarming news," said Flavio Di Giacamo of the International Organisation for Migration (IOM).
The Italian coastguard, which had despatched the Sea-Watch as part of its coordination of operations in the area, said around 3,000 people had been rescued in 20 operations and that seven bodies had been recovered.
This brings to around 5,400 the number of people rescued this week.
According to the UN, at least 3,654 people have died trying to cross the Mediterranean this year, the bulk of them on the Libya-Italy route.
Attempts at the dangerous crossing are continuing despite worsening weather as winter approaches, with more than 2,400 migrants rescued off Libya in total since Sunday.
The Italian interior ministry said Friday that more than 146,500 migrants had landed in Italy so far this year.
The number of arrivals is similar to that of the previous two years but the pressure on the country's reception facilities is mounting as it has become harder for migrants to move on to other EU countries.