By Stephanie Nebehay

GENEVA (Reuters) - U.N. investigators said on Tuesday they had expanded their list of suspected war criminals from both sides in Syria's civil war and the evidence was solid enough to prepare any indictment.

The U.N. inquiry has identified individuals, military units and security agencies as well as insurgent groups suspected of committing abuses such as torture and bombing civilian areas, it said in its report to the United Nations Human Rights Council.

Some 20 investigators have carried out 2,700 interviews with victims, witnesses and defectors in the region and by Skype in Syria, but have never been allowed to enter the country now in its fourth year of an increasingly sectarian conflict.

However, despite the accumulation of evidence, diplomats say it is unlikely Syria would be referred to the International Criminal Court (ICC) that tries war crimes suspects in The Hague any time soon.

As Syria has not signed the Rome statutes setting up the ICC, the U.N. Security Council would need to make the referral. Russia, supported by China, has shielded its ally Syria throughout the war, vetoing three U.N. resolutions that would have condemned President Bashar al-Assad's government and threatened it with possible sanctions.

"We do not lack information on crimes or even on perpetrators. What we lack is a means by which to achieve justice and accountability but this is not in our powers," Paulo Pinheiro, the chairman of the U.N. commission of inquiry on Syria, told a news conference.

The commission said the period of January 20 to March 10 was characterized by escalating hostilities between insurgent groups throughout northern and northeastern provinces as Islamist rebel strongholds came under attack.

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.

The mostly Sunni Muslim Insurgents seeking to topple Assad, whose minority Alawite sect is an offshoot of Shi'ite Islam, have used car and suicide bombs targeting civilian areas - also violations of international law, the commission said.

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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 before coming under attack by other armed groups such as the Islamic Front, it said.

CONFIDENTIAL LISTS

Four confidential lists of suspects have been drawn up, including the names of those responsible for hostage-taking, torture and execution, Pinheiro said.

"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," he said.

Carla del Ponte, a former U.N. war crimes prosecutor who is on the inquiry, said the investigators had gathered "objective evidence" including photographs and documents that could used by a prosecutor for any future ICC case.

"This commission has collected a lot of evidence that can be used tomorrow to prepare an indictment," she said.

"Referral to justice is an urgent, extremely urgent need, but as you know the Security Council cannot take the decision to refer to the ICC because of the veto."

Photos published by the Guardian newspaper in January appeared to confirm their previous findings on the government's systematic torture of detainees, the U.N. investigators said.

"I must assure you that the commission has taken these allegations very seriously, and we are investigating the evidence of torture, killing and starvation of detainees related to this case," Pinheiro added.

The independent team was set up in September 2011, months after the start of the revolt in which at least 140,000 people have been killed.

It has called repeatedly for the Security Council to refer Syria to the ICC prosecutor, a call endorsed again by Britain, the European Union, France and Switzerland on Tuesday.

Syrian ambassador Faysal Khabbaz Hamoui denounced the commission, in comments echoed by its ally Iran. "Referral to the ICC is a politicized and unlawful step as there are national judicial mechanisms available in Syria," he said.

Khabbaz Hamoui 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.

(Editing by Alison Williams)