Tension between Syrian opposition groups and Russia has escalated, with weeks of secret meetings in Turkey making little apparent progress on lifting the siege of eastern Aleppo.
Russia is a key backer of the Syrian President Bashar al-Assad and has been bombing opposition-held areas since September last year.
Turkey backs the Syrian anti-government fighters and has been acting as a mediator in the meetings.
Syrian opposition officials told Reuters news agency on Friday that Russia is not serious about the talks over a pause in the fighting.
The reports came as elite Syrian troops moved into east Aleppo in advance of a push into the most densely populated areas.
In preparation for street-by-street fighting, hundreds of fighters from the Republican Guard and Fourth Division arrived in Aleppo on Friday, the Syrian Observatory for Human Rights (SOHR) said.
Government forces have already "consolidated their control" over two eastern Aleppo districts and are pushing further to squeeze the opposition-held enclave, according to Rami Abdel Rahman, the SOHR head.
"After the recent advances, the regime is comfortably in control of half of former rebel territory in the city's east," he said late on Friday.
Earlier in the day, anti-government fighters had successfully rolled back government gains in Sheikh Saeed on Aleppo's southeastern outskirts.
Sheikh Saeed borders the last remaining parts of Aleppo still in opposition hands - a collection of densely populated residential neighbourhoods where thousands have sought refuge from advancing government forces.
The UN has warned that Aleppo risks becoming a "giant graveyard" in the middle of the Assad government's attempt to take over the city entirely.
John Kerry, US secretary of state, was expected to hold separate meetings with his Russian counterpart, Sergey Lavrov, and the UN's special envoy for Syria, Staffan De Mistura.
Absence of trust
Al Jazeera's Stefanie Dekker, reporting on Friday from Gaziantep on the Turkish-Syrian border, said: "A source at the meeting told us the Syrian fighters are not talking directly to the Russians, but through Turkish mediators."
She said "it is no secret that there's no trust between the two sides" though "it is Russia that holds all the cards.
"They are discussing some truce, some kind of cessation of hostilities, as well as sending aid and supplies into the eastern part of Aleppo".
Russia's proposals include expelling 200 fighters from Jabhat Fateh al-Sham, the armed group which changed its named from al-Nusra Front earlier this year when it said it cut ties with al-Qaeda.
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Jabhat Fateh al-Sham is one of the largest and most powerful groups fighting the Syrian government, and is considered by both the US and Russia to be a "terrorist" organisation.
Russia is also suggesting the creation of four "humanitarian corridors" to allow aid into besieged areas of eastern Aleppo.
"We know the Russians are very tough negotiators, that they are calling the shots," our correspondent said.
"The opposition is in an extremely difficult situation during these talks, given how much they have lost militarily on the ground."
Al Jazeera's Dekker said it is important to place these back-channel discussions in the proper context, as "realistically, whatever people say, it doesn't translate on to the ground.
"The reality is that fighting is [still] going on, the [Syrian government forces and its allies] are pushing from the east and from the south.
"There is still an active frontline, an active battle. People inside [eastern Aleppo] have told us that the situation there is dire, aid hasn't entered that area in five months. No humanitarian corridor has been set up. Aid groups would have to go through government territory, and many don't want to do that.
"There is no indication that, politically, [both sides] can come to any resolution."
Although Turkey, one of the key backers of Syrian opposition groups, has called for an immediate ceasefire, analysts contend that its priorities have shifted from Assad's removal to containing Kurdish groups seeking more territory on its border.
"It is clear that the interests of the Syrian people are third, fourth and fifth when it comes to all the countries involved in this war," our correspondent said.
Source: Al Jazeera News and Agencies