The United Nations is considering overseeing a Russian proposal to create humanitarian corridors for civilians who wish to leave besieged Aleppo, despite strong opposition from aid organisations.

Confidential documents seen by the Guardian detailing internal UN deliberations on the Kremlin’s proposal, described as “deeply flawed” by humanitarian agencies, reveal the contours of a debate inside an organisation that wants to provide assistance to suffering civilians in Aleppo but fears being seen as an accomplice in an onslaught that has left a quarter of a million civilians under siege.

A UN document outlining its position on the humanitarian corridors proposal says it will only implement the overseeing initiative if the warring sides agree to a ceasefire or pause in fighting.

Eastern Aleppo, which is under opposition control, has been under a nearly month-long siege by President Bashar al-Assad’s forces and Iranian-backed militias allied to his government.

The rebels have launched an offensive aimed at breaking the siege involving thousands of fighters, and have imposed a news blackout as they battle government troops backed by heavy Russian airstrikes.

If successful, the offensive could lead to the rebels imposing their own siege on western Aleppo, which is under government control and may have up to 1.5 million civilians and internal refugees living there.

The Russian proposal designates four areas that are intended as humanitarian corridors for civilians to flee eastern Aleppo.

Humanitarian agencies have several objections to the plan, but foremost among them is the concern that Assad and his allies could use the flight of civilians as justification for an even more indiscriminate campaign against eastern Aleppo, with the argument that those staying behind have chosen to aid “terrorists”, the regime’s catch-all term for the opposition.

Aid workers also see the plan as unworkable while intense fighting, some of the worst in Aleppo province since the start of the war, rages around the proposed corridors.

One of the two documents seen by the Guardian is a UN “position paper” that lays out the conditions that must be met before its humanitarian arms can take part in the plan. The other details the objections of international NGOs operating inSyria.

The first document says that while it sees the opportunity to move civilians from areas of “indiscriminate violence” as positive, UN agencies cannot take part in the proposal unless they are guaranteed humanitarian access to besieged civilians in the city.

The preconditions the UN has set before taking part include the proposal being overseen by a neutral third party, that residents who choose to stay in the city continue to have unhindered access to aid, and that all sides in the conflict pledge to not use indiscriminate and disproportionate force in the battle.

It also lays out conditions that demand the government not detain any civilians who leave the city, and that they ought to be able to choose where they want to relocate, potentially allowing them to resettle in other opposition-held areas.

In the second document, humanitarian agencies operating in Syria laid out their objections to the UN draft, saying that taking part in the corridors initiative risks “legitimising a move that would put civilians at risk”, and compromising the neutrality of aid organisations who might be seen as assisting a party to the conflict.

They also say the UN draft does not go far enough – that it should demand a full-fledged ceasefire rather than merely a humanitarian pause in the fighting, as well as allow civilians to relocate to areas under opposition control or seek asylum in Turkey.

“We would be concerned if any corridors are implemented that these are used to justify further violations … for example through providing ultimatums to civilians to leave or the implication that those who chose to remain are combatants or ‘terrorists’,” the document said.

The UN’s concerns will come as a blow to the efforts of Moscow, which has sought to alleviate criticism of the siege it has abetted in Aleppo by pointing to the humanitarian corridors as an alternative for civilians, though there is no evidence that any within opposition territory have taken it seriously.

Even the US, which has sought to more closely coordinate its military operations in Syria with the Kremlin, objected to the proposal, saying it risked ending further cooperation.

Asked about the UN’s position on humanitarian corridors and whether they could realistically be implemented, a spokesperson for the UN’s office for the coordination of humanitarian affairs referred to an earlier statement by Stephen O’Brien, the UN’s undersecretary for humanitarian affairs.

O’Brien’s statement said the security of any such corridors must be guaranteed by all parties and that people must be allowed to use them voluntarily. He also proposed 48-hour “humanitarian pauses” in the fighting to allow cross-border operations.

The spokesperson added that anyone using the corridors must be able to do so voluntarily and that safe routes must be available to deliver humanitarian aid.