BEIJING, July 28 -- Several senior UN officials called for sustained rescue efforts in the Mediterranean as the death toll of the shipwreck off Libya's western coast rose to 66.

With some 150 people rescued so far after the ship carrying around 400 immigrants capsized off the Libya coast, the number of deaths from the shipwreck is expected to increase.

The United Nations Refugee Agency (UNHCR) estimated that up to 150 lives would be lost as a result of the shipwreck, making it "the worst Mediterranean tragedy of the year."

All the immigrants drowned were women with only two girls rescued, said a survivor from Eritrea.

UN Secretary-General Antonio Guterres tweeted that he was "horrified" by the news, and called for "safe, legal routes for migrants and refugees."

His comments were echoed by the head of the UNHCR, Filippo Grandi, and the UN Children's Fund chief, Henrietta Fore, both of whom also called for an end to the arbitrary detention of refugees and migrants in Libya.

A spokesperson for the UNHCR said in a press briefing Friday that the large loss of life underscores the "terrible urgency" of the agency's repeated pleas to states to restore sea rescues and "help with alleviating the suffering of the thousands of refugees and migrants caught in the conflict in Libya."

Under the circumstance, two humanitarian groups, Doctors Without Borders (MSF) and SOS Mediterranean, said last week they have resumed migrant rescue operations in the Mediterranean Sea with a new vessel.

The non-governmental organizations had suspended operating their rescue boats in December, citing opposition by the Italian government, but resumed rescue operations a few days ago.

In March, the European Union significantly limited its search and rescue operation. In the past, navy and coast guard ships of several member states patrolled the Mediterranean.

That mission has largely been delegated to a partnership with the Libyan Coast Guard, which has been criticized by aid groups as brutal. MSF also argued that Libya is itself not a safe port and forces migrants to return to its often horrific migrant camps.

The UNHCR spokesperson reiterated the agency's request for additional help, such as further resettlement places, and other safe pathways out of Libya for vulnerable people, as well as measures to arrest and prosecute people traffickers.

The number of detainees is currently close to 10,000, according to UNHCR Libya Representative Roberto Mignone.

While the official camps are severely overcrowded, Mignone said the horrific abuse and torture that have been reported in the media take place in hideaways run by human traffickers, who continue extorting money from migrants before packing them onto unseaworthy vessels without enough fuel to make the crossing to Europe.

Before Thursday's shipwreck, 423 migrants had died this year trying to cross the central Mediterranean to Italy, according to the UNHCR.

The recent incident is expected to raise the death toll to over 600.