Some 115 people are missing and feared to have drowned after Libyan coast guard and local fishermen rescued 134 in a boat that capsized in the Mediterranean near Komas, east of Tripoli, according to Libyan navy officials on Thursday.
The UN refugee agency, UNHCR said earlier that some 150 people possibly drowned.
"The worst Mediterranean tragedy of this year has just occurred," UNHCR head Filippo Grandi wrote in a tweet.
According to Libyan Navy spokesman Ayoub Qassem, there were 250 people aboard the wooden boat, mainly from Eritrea and other sub-Saharan African countries, as well as Arab countries.
People from all over the African continent and the Arab world aim for Libya in order to travel across the Mediterranean to reach Europe.
This shipwreck brings the total of drowned migrant to more than 600 this year, according to spokesman Charlie Yaxley.
"Until we address the reasons why people take these dangerous boat journeys, sadly, this is unlikely to be the last tragedy like this that we see," he said.
He added that those who were rescued will be put in dangerous migrant detention centers in Libya and called for them not to be housed there.
"We know that inside these detention centres there's insufficient food, water, often unsanitary conditions, there have been widespread reports of human rights violations taking place," he said.
One detention centre was bombed earlier in July, killing more than 50 people inside. UNHCR said it has been closed, but those who are rescued by the sea are still sent there.
Politicians in the European Union have been accused of not dealing with the situation preferring to let people die as slaves or in inhumane conditions instead of dealing with the immigration issue.
More than one million refugees and migrants came to Europe in 2015.