By Michelle Nichols

UNITED NATIONS (Reuters) - The United Nations is ready to deliver aid into Syria's Aleppo, but needs commitments from all parties in the war - not just Russia - to abide by a 48-hour humanitarian truce, the U.N. aid chief, angered by lack of assistance to civilians, said on Monday.

Aleppo, Syria's most populous pre-war city and its commercial hub, has become the focus of fighting in the five-year-old civil war. Up to 2 million people on both sides do not have clean water after infrastructure was damaged in bombing.

Russia, which has been backing Syrian government forces with a bombing campaign, said on Thursday it supported the truce. The United Nations wants a weekly two-day halt in fighting to allow access to rebel-held eastern and government-controlled western Aleppo.

"While this (Russian) statement is positive, this cannot be a one-sided offer," the aid chief, Stephen O'Brien, told the U.N. Security Council. "Once we have the green light we can start to move assistance within 48 to 72 hours."

United Nations Secretary-General Ban Ki-moon warned last week of an unprecedented "humanitarian catastrophe" in Aleppo and urged Russia and the United States to quickly reach a deal on a ceasefire in the city and elsewhere in the country.

U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov have been discussing the issue. The United States, which supports some Syrian opposition groups, has been bombing Islamic State militants in Syria.

A crackdown by Syrian President Bashar al-Assad on pro-democracy protesters five years ago sparked a civil war, and Islamic State militants have used the chaos to seize territory in Syria and Iraq.

O'Brien said no aid had been delivered in August to nearly one million people in besieged and hard-to-reach areas because of fighting and Syrian government bureaucracy.

"I'm angry, I'm very angry," O'Brien said. "This callous carnage that is Syria has long since moved from cynical to sinful."

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Syrian U.N. Ambassador Bashar Ja'afari told the council that the Syrian government was not responsible for an air strike in rebel-held al-Qaterji neighborhood last week. A video of a dazed and bloodied Syrian boy - Omran Daqneesh - after he was pulled from the rubble of an air strike in that neighborhood last Wednesday shocked people around the world.

"The perpetrator must be elsewhere," Ja'afari said.

(Reporting by Michelle Nichols; editing by Grant McCool)