The migration crisis in the Mediterranean -- in which 800 people are feared to have died Sunday alone -- is a tragedy that needs a coordinated rescue response, the top U.N. official for refugees said Wednesday.

"Our ability to save lives at sea has to be guaranteed, because the current situation is a tremendous tragedy," U.N. High Commissioner for Refugees Antonio Guterres said at a briefing at the Organization of American States in Washington.

French President Francois Hollande on Tuesday called for Europe "to go much further" in tackling the migrant crisis after a series of deadly disasters in the Mediterranean.

"The European summit on Thursday cannot be an ordinary summit where we take ordinary decisions," Hollande said ahead of the urgent meeting called after the latest migrant shipwreck saw 800 people drown in the Mediterranean.

The foreign minister of Libya's internationally recognized government condemned on Tuesday "death smugglers" who organized the doomed voyage to Europe that left an estimated 800 dead in a shipwreck.

"The migrants left from areas under the control of militia. We condemn the acts of these death smugglers," Mohamed al-Dayri told AFP in a telephone interview from Indonesia where he is attending an Asia-Africa summit.

Australia's prime minister urged European leaders on Tuesday to adopt tougher border control measures in the wake of the feared drowning of as many as 900 asylum seekers whose boat capsized off the coast of Libya.

Prime Minister Tony Abbott, whose government implemented a strict policy of turning back asylum seekers' boats in a bid to discourage them from trying to reach Australia, called the latest Mediterranean crisis a "terrible, terrible tragedy" and suggested Europe follow Australia's lead to ensure it's not repeated.

Ethiopia began three days of national mourning on Tuesday, with joint Christian and Muslim prayers for more than 20 Ethiopian Christians killed by Islamic State militants in Libya.

The murders have horrified Ethiopians and sparked global condemnation, including from Pope Francis who expressed his "great distress and sadness".

A bomb exploded outside Spain's embassy in the Libyan capital on Monday night without causing injuries, a security source told Agence France Presse, an attack later claimed by the Islamic State (IS) group.

"Islamic State extremists concealed an explosive device near an exterior wall of the Spanish embassy building in Tripoli, which caused some material damage to it and to neighboring buildings," local official Issam Naas told AFP.

The captain of a migrant boat that capsized in the Mediterranean's deadliest disaster in decades was arrested Tuesday on suspicion of causing the deaths of an estimated 800 people.

Prosecutors in the Sicilian city of Catania said they believed Tunisian national Mohammed Ali Malek, 27, was responsible for steering mistakes and the reckless overcrowding which led to the horrifying shipwreck off Libya on Sunday.

Italy is studying the possibility of mounting "targeted interventions" against Libya-based people smugglers behind a huge surge in the numbers of migrants crossing the Mediterranean, Prime Minister Matteo Renzi said Monday.

"The hypothesis of military intervention (to stabilize Libya) is not on the table... but what is possible are targeted interventions to destroy a criminal racket," Renzi said at a press conference with his Maltese counterpart Joseph Muscat.

Ethiopia will hold three days of national mourning for Ethiopian Christians killed by Islamic State militants in Libya, the government said Monday.

The mourning period will begin Tuesday after its official launch by parliament, Communications Minster Redwan Hussein told AFP. 

French President Francois Hollande on Monday condemned the killing of at least 28 Ethiopian Christians in Libya, urging a peace deal to restore order in the conflict-ridden nation.

Hollande said in a statement he was outraged at the "abominable" murder shown in a video by the Islamic State (IS) group released Sunday.