Calls for an independent probe mounted Wednesday after more than 40 migrants were killed in an air strike on a detention centre in Libya that the UN said could constitute a war crime.
UN chief Antonio Guterres denounced the "horrendous" attack and demanded "an independent investigation", his spokesman said, as the Security Council was to hold urgent talks about the situation in Libya.
The European Union called the attack "horrific" and urged the UN to launch a probe as Libya's internationally-recognised government blamed the deadly assault on commander Khalifa Haftar.
Bodies were strewn on the floor of a hangar in the Tripoli suburb of Tajoura, mixed with the belongings and blood-soaked clothes of migrants, an AFP photographer said.
"There were bodies, blood and pieces of flesh everywhere," a survivor, 26-year-old Al-Mahdi Hafyan from Morocco, told AFP from his hospital bed where he was being treated for a leg wound.
Tuesday night's strike left a hole around three metres (10 feet) in diameter at the centre of the hangar, surrounded by debris ripped from the metal structure by the force of the blast.
At least 44 people were killed and more than 130 severely injured, the UN said.
Guterres recalled that the United Nations had shared the coordinates of the Tajoura detention centre east of Tripoli with the warring sides to ensure that civilians sheltering there were safe.
The UN chief "condemns this horrendous incident in the strongest terms," said a statement from his spokesman.
He "calls for an independent investigation of the circumstances of this incident, to ensure that the perpetrators are brought to justice, noting that the United Nations had provided exact coordinates of the detention centre to the parties."
His envoy to Libya Ghassan Salame said earlier: "This attack clearly could constitute a war crime, as it killed by surprise innocent people whose dire conditions forced them to be in that shelter."
"The absurdity of this ongoing war today has led this odious bloody carnage to its most hideous and most tragic consequences," Salame said, noting it was the second time the facility had been attacked.
Around 600 migrants and refugees were held in the Tajoura detention centre, the head of the compound Noureddine al-Grifi said, adding that people were wounded in another hangar.
The Tripoli-based Government of National Accord (GNA) denounced the attack as a "heinous crime" and blamed it on the "war criminal Khalifa Haftar".
The European Union also called for an independent probe.
"Those responsible should be held to account", EU diplomatic chief Federica Mogherini, enlargement commissioner Johannes Hahn and migration commissioner Dimitris Avramopoulos said in a statement.
Several countries also called for an international investigation, including GNA ally Turkey which denounced a "crime against humanity" and Qatar which said the attack "may amount to war crimes".
Haftar, who controls much of eastern and southern Libya, launched an offensive to take the capital in April.
The UAE, Egypt and Saudi Arabia are seen as Haftar's key supporters while he accuses Turkey and Qatar of supplying weapons to his rivals.
The GNA accused pro-Haftar forces of having carried out a "premeditated" and "precise" attack on the migrant centre.
There has been no immediate claim of responsibility for the attack, but pro-Haftar media reported Tuesday night a "series of air raids" in Tripoli and Tajoura.
The suburb of Tajoura, which has several military sites belonging to pro-GNA armed groups, is regularly targeted in air raids by Haftar's forces.
Migrants 'at risk'
"Migrants and refugees must NOT be detained; civilians must NOT be a target; Libya is NOT a safe place of return" for migrants and refugees, the head of the UN refugee agency (UNHCR), Filippo Grandi tweeted.
UNHCR spokesperson Charlie Yaxley said in Geneva the agency had asked to have the centre evacuated a few weeks ago after "a near miss from a similar air strike".
The centre was thought to have been used to store weapons, he added.
The UN's mission in Libya has said around 3,500 migrants and refugees held in detention centres near the combat zone are at risk.
Wracked by chaos since the 2011 uprising against dictator Moamer Kadhafi, Libya has become a major conduit for migrants seeking to reach Europe.
Rights groups say migrants face horrifying abuses in Libya, which remains prey to a multitude of militias vying for control of the oil-rich country.
The plight of migrants has worsened since Haftar launched the offensive against Tripoli.
More than 700 people have been killed and 4,000 wounded since the assault began in early April, while nearly 100,000 have been displaced, according to UN agencies.
The two rival camps accuse each other of using foreign mercenaries and enjoying military support, especially air, from foreign powers.