Russia and the Syrian government have said they will open up a route to give besieged civilians, as well as rebels willing to surrender, a way out of the northern city of Aleppo.

Backed by Russian air strikes, the Syrian army has been increasingly encircling the divided city, targeting opposition fighters every day and cutting off all supply routes into the rebel-held part of Aleppo.

In a televised speech on Thursday, Russian Defence Minister Sergei Shoigu announced joint plans between Moscow and Damascus to "open three humanitarian corridors in order to help civilians who were kept hostage by the terrorists as well as the fighters who want to lay down their arms".

Syrian President Bashar al-Assad has also offered a general amnesty for rebels who want to give up their weapons and surrender to authorities over the next three months.

The Syrian government has reportedly been dropping leaflets detailing safe passage routes for more than 300,000 trapped civilians in Aleppo. In text messages to civilians and fighters, the army also urged residents on Tuesday to "join the national reconciliations and expel the mercenary outsiders from the areas where the citizens reside", according to the state-run news agency SANA.

READ MORE: Syrian army 'cuts off last supply route to east Aleppo'

Al Jazeera's Mohamed Jamjoom, reporting from Gaziantep on the Turkish side of the Syrian-Turkish border, said, however, that Aleppo civilians were suspicious and distrustful of the proposed plan.

"Everybody we've spoken to on the ground in Aleppo says that not only do they not trust either the Syrian regime or the Russians to implement this plan - and that the rebel forces will be safe if they surrender or if civilians flee through these corridors - but they also say it does not make sense to them that this would be carried out in this way."

Jamjoom added that many Syrians said they would have preferred such a plan to be carried out by the UN or aid agencies.

"They feel that if they stay inside Aleppo, the siege is only going to get worse and the humanitarian crisis is only going to increase. They say if they leave, they fear they will be treated harshly by Syrian forces," Jamjoom said.

"They don't know what to do."

Commenting on the Syrian-Russian proposal, Staffan De Mistura, the UN special envoy for Syria, said the UN, "like everyone else", was not consulted on the matter beforehand - but that it was willing to find ways to coordinate with Russia on the plan.

"The city is de facto besieged, because it is almost completely militarily encircled," said de Mistura, adding there were only two to three weeks of supplies left.

"The clock is therefore ticking, there is no doubt about that."

The Russian government said it welcomed international aid organisations, which operate in Syria, to join the operation.

Humanitarian groups have warned of a major catastrophe if the siege on the rebel-held east of Aleppo continues, with medical facilities, bakeries and warehouses reportedly being hit.

Some 300,000 residents are trapped in the eastern part of the city, according to the UN.

READ MORE: Air strikes pound Aleppo as UN eyes new Syria talks

Once Syria's largest city, Aleppo has been roughly divided between government control in the west and rebel control in the east since mid-2012.

Assad has issued amnesty offers several times in the past in the course of Syria's civil war, now in its sixth year. The offer is largely seen by opposition fighters as a publicity stunt and psychological warfare against the rebels.

De Mistura said this week he hopes peace talks aimed at ending more than five years of brutal conflict could resume at the end of August.

In the past few days, dozens of Syrians have been reportedly killed in separate attacks across the divided city.

The Syrian conflict began as a mostly unarmed uprising against Assad in March 2011, but it quickly escalated into a full-blown civil war between the government, armed opposition fighters and hardline groups.

More than 280,000 Syrians have been killed in the conflict.