The United States is trying to determine whether a Russian plan for a humanitarian operation in Syria is sincere, Secretary of State John Kerry said on Friday, adding that if it proves a "ruse" it could ruin cooperation between Moscow and Washington.

The 250,000 civilians trapped for weeks inside the besieged rebel-held sector of Aleppo have so far stayed away from "safe corridors" that Moscow and Damascus promised for those trying to escape the most important opposition stronghold in the country.

Syrian President Bashar al-Assad's government and its Russian allies declared a joint humanitarian operation for the besieged area on Thursday, bombarding it with leaflets telling fighters to surrender and civilians to leave.

US officials have suggested the plan may be an attempt to depopulate the city so that the Syrian army can seize it. The Syrian opposition called it a euphemism for forced displacement of the inhabitants, which it said would be a war crime.

Aleppo, Syria's biggest city before the war, has been divided since 2012 into government and rebel sectors. Retaking it would be the biggest victory for Assad in five years of fighting, and demonstrate the dramatic shift of fortunes in his favour since Moscow joined the war on his side last year.

This would also be an embarrassment for Kerry, who has led a diplomatic initiative with Moscow aiming to let the Cold War superpower foes cooperate against Islamist militants and restore a ceasefire for the wider civil war which collapsed in May.

Asked about the Russian operation, Kerry said Washington was still unsure of Moscow's intent: "It has the risk, if it is a ruse, of completely breaking apart the level of cooperation."

"On the other hand, if we're able to work it out today and have a complete understanding of what is happening and then agreement on the way forward, it could actually open up some possibilities," he added, saying he had spoken with Moscow twice in the past 24 hours to try to clarify what Russian intentions.

The White House also voiced its doubts. "Given their record on this, we're sceptical, to say the least," White House spokesman Eric Schultz said at a news briefing.

Turning point

The fate of Aleppo in the coming weeks has the potential to be a turning point in a seemingly endless, multi-sided civil war that has killed hundreds of thousands of people, driven millions from their homes and drawn in most world and regional powers.

Pro-government forces with Russian backing have advanced in the three months since the ceasefire collapsed, and imposed a siege on the rebel-held sector of Aleppo since early July when they closed the main road out of the city.

The United Nations says food will run out within weeks for the people trapped inside, and has been trying to negotiate regular pauses in the fighting to allow humanitarian access.

So far, the safe zones have not been used. Syrian state television accused the rebels of preventing civilians from leaving, which rebels deny. A state TV reporter in Aleppo said reception centres with health and food supplies had been set up around Aleppo to receive civilians, but so far few had come through because rebel fighters were threatening them.

The main opposition High Negotiations Committee (HNC) says it believes the aim is to cleanse the area so government forces can capture it.

"The world must not allow Russia to get away with disguising its assault on Aleppo with deceitful talk about humanitarian 'corridors.' Be clear – these 'corridors' are not for getting aid in, but driving people out. The brutal message to our people is – 'leave or starve'," HNC member Bassma Kodmani said.

Privately, US officials fear the Russian proposal masks the real intent of its Syrian ally, to separate boys and men from the rest of the population, claim they are terrorists and either imprison or execute them, "as Assad and his father have done repeatedly at least since 1982", said one official, discussing Washington's analysis on condition of anonymity.

"Why would you evacuate a city that you wanted to send humanitarian aid to?" asked a second official.

Ghaith Yaqout al-Murjan, an activist in Aleppo, told Reuters civilians were avoiding the corridors as they were still unsafe: "There are people who want to leave because they can no longer bear the shelling by helicopters, jets, barrel bombs...The rebels are not holding anyone if they want to leave."

"You are talking about the need to walk a kilometre in a battle where you are at risk of being hit from two sides."

The United Nations, which hopes to resume peace talks in August, has been circumspect since Russia announced the humanitarian operation, saying the proposal for safe corridors out could be helpful, but only if combined with humanitarian access for those who do not want to leave.

Urgent need for improvement

UN mediator Staffan de Mistura said there was "urgent need for improvement" in the plan, but that Moscow appeared to be open to suggestion.

Regular pauses for humanitarian access were necessary, he added, and the United Nations should be involved in managing any safe exit routes.

The International Committee of the Red Cross said safe corridors could only work if agreed by the warring parties, and there was no sign of any such agreement in Aleppo.

"The ICRC is not a big fan of humanitarian corridors, because it always runs the risk that there is the concept of safe areas, and everything outside those safe areas becomes an area of non-respect for international humanitarian law," ICRC Middle East regional head Robert Mardini told reporters in Geneva.

Aid agencies say civilians have been unable to leave through the safe corridors because of fighting, and that the situation inside the besieged city is becoming increasingly perilous.

Save the Children quoted a doctor describing dire conditions of constant bombardment and mass casualties inside the city.

"Imagine the emergency room in any of the field hospitals doesn't have more than five or six beds, and when responding to a massacre they receive up to 30-40 injured at the same time," the doctor said in a statement released by the aid group.

At one bombing site, "a child less than 10 years old ran to me shouting 'sir, please put my arm back'. His left arm was amputated and he held it with his right hand. He was begging me to put it back, and this is only one of so many tragedies that we see."

Separately, Save the Children said one of its maternity hospitals had been bombed by government forces in Idlib, another province where there has been fighting since a ceasefire collapsed in May.