United Nations human rights investigators have added to their list of suspected war criminals from both sides of the Syrian conflict and the evidence was solid enough to prepare any indictment after a new round of atrocities in recent weeks.

The UN inquiry identified military units and security agencies as well as opposition groups suspected of committing abuses, Paulo Pinheiro, the chairman of the inquiry said on Tuesday.

Four confidential lists of suspects on both sides have been drawn up to date.

"This 'perpetrators list', as we call it, contains names of persons criminally responsible for hostage-taking, torture and executions," Pinheiro told the Human Rights Council.

"It also contains names of the heads of intelligence branches and detention facilities where detainees are tortured, names of military commanders who target civilians, airports from which barrel bomb attacks are planned and executed, and armed groups involved in attacking and displacing civilians."

In its updated report, the UN commission of inquiry on Syria said the period of January 20 to March 10 was marked by escalating infighting within opposition groups throughout northern and northeastern provinces.

Syrian government forces dropped barrel bombs on Aleppo and other cities, it said, causing extensive civilian casualties in areas with no clear military target. They also severely tortured detainees, according to the report.

Mass executions

Opposition groups seeking to topple President Bashar al-Assad have used car and suicide bombs targeting civilian areas, also violations of international law, the UN said.

ISIL used the Children's Hospital building in Aleppo as its headquarters and as a detention facility. Later, fighters from another group discovered an 'execution field' near the hospital.

"In the days and hours prior to attack, ISIL fighters conducted mass executions of detainees, thereby perpetrating war crimes.

The number killed as well as allegations of mass graves connected to these executions remain under investigation," it added.

The independent team of more than 20 investigators, set up in September 2011, months after the start of the revolt now in its fourth year, includes former UN war crimes prosecutor Carla del Ponte.

'Highly-politicized'

It has called repeatedly for the Security Council to refer the situation in Syria to the prosecutor of the International Criminal Court (ICC), a call endorsed again by Britain, the European Union (EU), France and Switzerland on Tuesday.

"It is this volume of testimony that will be the enduring legacy of the Commission: an archive of Syrian voices and a resource for future prosecutions," Pinheiro said.

Syrian ambassador Faysal Khabbaz Hamoui took the floor to denounce the commission.

"Referral to the ICC is a politicized and unlawful step as there are national judicial mechanisms available in Syria," he said.

He accused the commission of working for the political agendas of countries that are supporting the opposition- naming the United States, Qatar, Turkey and Saudi Arabia.

Iran, which supports Assad, condemned extreme violence committed by "terrorist and extremist groups" in Syria. The Iranian delegation said the move to refer its ally to the ICC constituted a "highly-politicized and illegitimate incitement".

PHOTO CAPTION

A Syrian man evacuates a child found in the rubble of a residential building reportedly hit by an explosives-filled barrel dropped by a government forces helicopter on March 18, 2014 in Aleppo.

Aljazeera