(CNN) Rebel groups in Syria's war-ravaged Aleppo put up a united front on Thursday in a final effort to prevent regime forces from seizing the whole city.
Rebels in eastern Aleppo have held ground in the Sheikh Saeed neighborhood as they continue to clash with regime troops, in an attempt to protect the southern parts of the enclave south after Syrian forces made sweeping territorial gains in the north.
Syrian troops backed by militia gunmen loyal to President Bashar al-Assad entered eastern Aleppo on Saturday and have seized the entire northeast. They are now in control of more than 20% of eastern Aleppo.
Rebel groups, which have controlled eastern Aleppo for more than four years, on Thursday announced a new alliance, the Aleppo's Army coalition, under the leadership of a rebel named Abu Abdul Rahman Nour.
In a statement, the coalition said it aimed "to save Aleppo and its people." It is seen a sign that they may be willing to negotiate with the regime under one umbrella.
At least 40 people were killed in shelling on Thursday as they were trying to flee rebel-held areas, the Aleppo Media Center said. The death toll has reached more than 600 since Saturday, according to various activist groups.
The regime's ground operation has been backed by intense airstrikes that have caused severe damage to parts of eastern Aleppo. On Thursday's rebels won a short respite from the strikes, as heavy rain prevented warplanes from attacking.
If the regime takes all of Aleppo -- the last urban rebel stronghold in Syria -- it would mark a turning point in the war, putting Assad's government back in control of all four major cities, making a political opposition less likely.
The 15-member UN Security Council held an emergency meeting in New York on Wednesday, at which the US continued to blame Russia for backing the Syrian regime's "atrocities" in eastern Aleppo.
Russia is the most powerful ally of Assad's regime and has carried out airstrikes since September 2015 to prop up the embattled leader. But Moscow has tried to distance itself from the current assault, denying it is currently bombing the city.
Russia has regularly used its veto power as a permanent member of the council to shoot down resolutions on Syria. China, also a permanent member, has also voted against some of them.
At the heart of the deadlock is that Russia and Syria consider all rebel groups in the country terrorist groups, while some of the very same rebels are considered moderate by the US and have been armed and supported by Washington to fight ISIS.
UN humanitarian chief Stephen O'Brien made yet another plea for the council to adopt a resolution put forward by Egypt, New Zealand and Spain for a 10-day pause in the fighting and bombing to allow desperately needed aid into the war zone.
"For the sake of humanity, we call on, we plead with the parties and those with influence to do everything in their power to protect civilians and enable access to the besieged part of eastern Aleppo before it becomes one giant graveyard," O'Brien told council members during the meeting.
Since the ground operation began on Saturday, at least 27,000 people have fled eastern Aleppo neighborhoods, UN humanitarian adviser Jan Egeland told journalists Thursday.
But around 200,000 people remain, and aid is only reaching those who have fled to the government-controlled west and Kurdish-controlled areas, Egeland said.
The UN had hoped Russia and Syria would allow a pause in fighting to gain access to the east, he said, adding that 400 people there were in desperate need of medical evacuation.
The UN has been looking for a concrete guarantee from Russia and Syria that their aid trucks will be given safe passage.
Gareth Bayley, Britain's special representative for Syria, said just five of 25 planned aid convoys were able to make deliveries in November.
Russian President Vladimir Putin said on Thursday that Moscow was prepared to cooperate with President-elect Donald Trump and his administration on fighting terrorism.
The Syrian war has also raised tensions between Russia and Turkey.
Turkish President Recep Tayyip Erdogan ruffled feathers in Moscow after he said on Tuesday that Ankara sent its forces into Syria to help end "the rule of the cruel Assad." Turkey backs several anti-Assad rebel groups in Syria.
Russian Foreign Minister Sergey Lavrov met his Turkish counterpart, Mevlut Cavusoglu, on Thursday, and the two stood firm on their positions in the conflict, but agreed that a ceasefire was necessary.