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 / Updated  / Source: Reuters

The United Nations said Thursday it was trying to broker a temporary ceasefire in Aleppo for humanitarian aid, warning time was running out for up to two million trapped civilians.

It came after Russia earlier said there would be daily three-hour ceasefires – but witnesses on the ground told Reuters Thursday there had been no pause in fighting.

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U.N. humanitarian adviser Jan Egeland said a 48-hour pause was required to ensure safe deliveries, adding: "What is the new and positive thing today is that the Russian Federation said they would like to sit down with us and the other co-chair [the United States] to discuss how the U.N. proposal could be implemented. We are hopeful that will lead to something."

A rebel fighter sits with his weapon in the artillery academy of Aleppo on Monday.AMMAR ABDULLAH / Reuters

It came as officials investigated reports that chlorine gas had been dropped on Aleppo, killing civilians and causing breathing difficulties in others.

Hamza Khatib, the manager of Al Quds hospital in Aleppo, told a Reuters photographer the hospital had recorded four deaths from gas poisoning and 55 injuries. Seven people were still receiving hospital treatment.

"There is a lot of evidence that it actually did take place", U.N. Special Envoy for Syria Staffan de Mistura told reporters, adding that the attack would amount to a war crime if confirmed.

A Syrian military official told Reuters the gas attack claims had been fabricated by militants and that the Syrian army would never use chemical weapons.

Aleppo is split into rebel and government controlled areas. The rebel-held east, where about 250,000 people are thought to be living, came under siege in early July after government forces cut the Castello Road, the main supply route into the district.

Food supplies, infrastructure and medical services are immensely strained for trapped civilians.

Russian and Syrian warplanes have bombarded eastern Aleppo and other rebel-held areas of Syria daily for months and the U.S. suggested the humanitarian corridors plan may have been an attempt to depopulate the city so that the Syrian army could seize it.

Separately, Syrian activists said at least 20 civilians have been killed in airstrikes on Raqqa, the de facto ISIS capital in northern Syria.

The Britain-based Syrian Observatory for Human Rights also reported the airstrikes, saying 24 civilians were killed, along with six others whose affiliation and identities could not yet be confirmed.