The U.N. inquiry has repeatedly called for the Security Council to refer the situation in Syria to the prosecutor of the International Criminal Court

By Stephanie Nebehay

GENEVA, March 18 (Reuters) - United Nations human rights investigators have added to their list of suspected war criminals from both sides in the Syrian civil war after a new round of atrocities in recent weeks, its head said on Tuesday.

The U.N. inquiry has identified military units and security agencies as well as insurgent groups suspected of committing abuses, Paulo Pinheiro told the Human Rights Council.

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," said Pinheiro, a Brazilian chairing the inquiry.

"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 update report, the U.N. commission of inquiry on Syria said the period of Jan. 20 to March 10 was marked by escalating hostilities between insurgent groups throughout northern and northeastern provinces as Islamist rebel strongholds came under attack.

Syrian government forces have dropped barrel bombs on Aleppo and other cities, causing extensive civilian casualties in areas with no clear military target, and severely tortured detainees.

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

Fighters from al Qaeda splinter group the Islamic State of Iraq and the Levant, also known as ISIS, executed detainees, including civilians and captured soldiers, in Aleppo, Idlib and al Raqqa in the hours and days before coming under attack by other armed groups such as the Islamic Front, it said.

ISIS 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, ISIS 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.

POLITICISED

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

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 politicised 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 rebels - 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-politicised and illegitimate incitement".

(Reporting by Stephanie Nebehay; Editing by Angus MacSwan)

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