Nearly 150 migrants are feared dead after boats carrying them capsized in the Mediterranean off the coast of Libya on Thursday, a United Nations High Commissioner for Refugees official told AP. Two boats carrying around 300 migrants capsized around 120 km east of Libya’s capital Tripoli, said Ayoub Gassim, a spokesperson for Libya’s coast guard.

About 137 migrants had been rescued and sent back to Libya and one body has been recovered by the coast guard so far, Gassim said. “We estimate that 150 migrants are potentially missing and died at sea,” said Spokesperson for the United Nations High Commissioner for Refugees Charlie Yaxley.

United Nations High Commissioner for Refugees Filippo Grandi described the incident as the “worst Mediterranean tragedy of this year”, urging European countries to restart rescue operations in the Mediterranean, which was ceased after a European Union decision. He also appealed to stop migrant detentions in Libya, and said that safe pathways out of the country were needed “before its too late for many more desperate people”.

Earlier this week, Libyan coast guard officials intercepted over 30 migrants and shifted them to a detention centre near Tripoli where an airstrike had killed more than 50 people on July 3. At least 6,000 migrants from Eritrea, Ethiopia, Somalia, Sudan and other countries have been held in detention centres by the government.

UNHCR said that 164 migrants, travelling from Libya to Europe, have died since the beginning of this year, a number fewer than previous years. However, the United Nations said that the journey has become more perilous with one out of four dying at sea.

Following the uprising in Libya that killed its dictator Muammar Gaddafi in 2011, migrants from the country have attempted to cross the seas for a better life in European countries. However, this has also fuelled trafficking and have led to exploitation by armed groups of migrants, according to The Telegraph.