An agreement has been reached with Russia to extend a ceasefire in Syria to Aleppo province, including the besieged city of Aleppo, the US State Department said.

The expansion went into effect on Wednesday just after midnight in Damascus (02:00 GMT), the State Department said, noting an "overall decrease in violence" since then despite some continued fighting.

"Since this went into effect today at 00:01 in Damascus, we have seen an overall decrease in violence in these areas," spokesman Mark Toner said.

The Syrian army confirmed the ceasefire, saying there would be a "regime of calm" in Aleppo for 48 hours, Russian ambassador to the UN Vitaly Churkin told the UN Security Council.

Al Jazeera's Rosiland Jordan, reporting from Washington DC, said that the announcement was delayed because "officials wanted to see how long it would take for the ceasefire to come into effect.

"They decided to announce it now because they believe that the ceasefire is holding."

The US is coordinating with Russia to finalise monitoring efforts for the ceasefire and calls on all parties to abide by the agreement.

"We look to Russia as a co-chair of the International Syria Support Group to press for the Assad regime's compliance with this effort, and the United States will do its part with the opposition," the State Department statement said.

'War crimes, crimes against humanity'

The United Nations Humanitarian Affairs chief Stephen O'Brien told the UN Security Council that the killing of civilians in Aleppo "cannot and will not be forgotten", warning that perpetrators will be held accountable.

O'Brien gave a rundown of deadly incidents in which residential areas, medical facilities and ambulances were targeted by government war planes and non-state shelling.

"Some of these acts amount to war crimes and crimes against humanity," O'Brien said in New York.

Activists and rebels said at least 250 people have been killed in the fighting in Aleppo in the past 10 days.

On Wednesday, dozens of people were killed in a day-long battle in western Aleppo that was still going on intermittently, the UK-based Syrian Observatory for Human Rights and sources on both sides said.

Sources loyal to the Syrian government gave conflicting accounts of the outcome of the battle that began early on Tuesday in and around the Jamiat al-Zahraa area of western Aleppo.

A rebel told AP news agency that fighters had managed to take some ground from the government side, while the army said the attack was repelled.

Aid delivery blocked

UN humanitarian adviser Jan Egeland said on Wednesday that the Syrian government had refused UN demands to deliver aid to hundreds of thousands of people, including many in violence-torn Aleppo.

Aleppo has seen fierce fighting between the warring sides in the past two weeks after government forces began targeting the city.

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"We seem to be having new possible besieged areas on our watch. We are having hundreds of relief workers unable to move in Aleppo," Egeland told reporters on Wednesday after chairing a weekly meeting of nations supporting the Syria peace process.

"It is a disgrace to see that while the population of Aleppo is bleeding, their options to flee have never been more difficult than now."