By Stephanie Nebehay

GENEVA (Reuters) - The U.N. Human Rights Council said on Friday it would identify the perpetrators of war crimes in the besieged Syrian city of Aleppo and launched a special inquiry into the use of starvation and air strikes there, as well as increased "terrorist" attacks.

Zeid Ra'ad al-Hussein, United Nations High Commissioner for Human Rights, had earlier called for major powers to put aside their differences and refer the situation in eastern Aleppo to the International Criminal Court.

He said that the siege and bombing there constituted "crimes of historic proportions" that have caused heavy civilian casualties amounting to war crimes.

Zeid did not name Russia or the Syrian air force, whose jets have attacked the rebel-held districts of Aleppo for weeks, but his reference was clear.

"Armed opposition groups continue to fire mortars and other projectiles into civilian neighborhoods of western Aleppo, but indiscriminate air strikes across the eastern part of the city by government forces and their allies are responsible for the overwhelming majority of civilian casualties," he said in a speech to a special session of the U.N. Human Rights Council.

Such violations constituted war crimes and if there was intent to commit them as part of a widespread or systematic attack against civilians, they would amount to crimes against humanity, he said.

"The violations and abuses suffered by people across the country, including the siege and bombardment of eastern Aleppo, are simply not tragedies; they also constitute crimes of historic proportions," Zeid told the Geneva session.

Britain's resolution to launch the inquiry was adopted by 24 states in favor with 7 against, including Russia and China, and 16 abstentions. The investigation, to be conducted by an existing U.N. commission of inquiry on Syria led by Paulo Pinheiro, is to issue its findings by March.

John Fisher of Human Rights Watch said the "decisive action" had "sent a clear message that illegal attacks on civilians must end and that those responsible will be held to account".

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Russia has denied any deliberate targeting of civilians and says it is combating terrorists.

"These are the worst terrorists humanity has ever known. Nusra is not different from Daesh, it is the same wahhabi extreme fanaticism, they only believe in killing," Syria's ambassador Hussam Aala told Reuters.

"This is the reality we face on the ground," he said.

CEASEFIRE

Russia and Syria said that their forces were observing an 11-hour truce in Aleppo to allow evacuation of the wounded and for civilians to leave.

Britain's junior Foreign Office minister Tobias Ellwood told reporters that the Russian pause was "being used simply for them to regroup and further their own stranglehold over Aleppo".

Referring to the air strikes, he said: "This is shameful and it is not the action or leadership that we expect from a P5 (permanent member of the U.N. Security Council) nation."

Ted Allegra, deputy U.S. ambassador, said the Russian and Syrian assault had killed 400 people, including 100 children.

"These shocking acts in Aleppo beg for an appropriate investigation and those who commit them must be held accountable," he said.

Russian Ambassador Alexey Borodavkin accused Britain and its allies of "trying to save terrorists from being the target of strikes, allowing them to regroup and continue their barbaric acts".

An 11-hour unilateral ceasefire in Aleppo was "allowing civilians and those fighters who lay down their weapons to leave" the city, he said.

(Additional reporting by Tom Miles; Editing by Louise Ireland)