The text message sent Sunday to residents warned the sick and wounded to leave before a "strategically planned assault using high precision weapons occurs within 24 hours."
It was most likely sent by the Syrian government as the regime is the only party believed capable of sending a text en masse, and has been in a months-long bid to regain control of the area held by rebels.
Despite the threat and a worsening humanitarian situation, many are refusing to leave.
The Syrian regime, along with Russia, have obliterated parts of the city's east in recent months, and although they say they have created humanitarian corridors out, many residents do not trust that they will make it to safety on the other side.
Others are refusing out of principle.
"My wife is afraid, my little students are afraid of what's going on -- we should be afraid," English teacher and activist Abdulkafi Alhamdo told CNN on Monday.
"We want the world to know we are here to stay, not ... because we could not go out. We could have gone out many times -- death might come, but death might come anywhere. But our freedom cannot be found anywhere," he said.
'We are running out of everything'
In the text message warning, rebels were also given an ultimatum to lay down their arms and renounce their leadership, or be killed.
The 24-hour period mentioned in the text has passed, and while witnesses told CNN they saw warplanes swirling the city, there have so far been no reports of airstrikes.
But people believe it is still coming.
Resident Fatemah Alabed doesn't want to leave, but the lack of basic needs is making life near impossible.
"We are running out of everything," she told CNN, adding that she had only small stocks left of rice, pasta and cracked wheat.
"We planted some herbs on our rooftop, things like parsley and mint, also some spinach."
Her family gets running water sporadically, every week or two, but they often resort to water from wells that gives her children stomach pains.
A 'killer' winter is coming
And as goods run low, prices are skyrocketing.
The UN warned last week that eastern Aleppo was on the brink of starvation ahead of a "killer" winter.
Residents there told CNN that their food stocks were running out, and that markets that once sold fruit and vegetables were now empty. A kilogram of meat, they said, costs around $40, a price that most in Aleppo simply cannot pay.
The last significant aid delivery was in July, and the area is extremely low on medicine and much-needed fuel to run hospital generators and ambulances.
Gas cylinders to cook or heat a home are at $200, while a pack of cigarettes now costs $100, an activist from the Aleppo Media Center told CNN.
"The situation is the worst it has ever been in (eastern) Aleppo," the activist said.
"People cannot find anything to eat. Even bread is not widely available."
Russia flexes military muscle
Meanwhile, Russia has moved an aircraft carrier and battle cruiser to the Syrian coast, the ships' captains told Russian TV station Vesti over the weekend, in a boost to Moscow's military presence in the war-torn country.
Russia is the most powerful sponsor of Syrian President Bashar al-Assad's regime, and its air power has been a key factor in helping the government solidify its control over Aleppo.
Assad has insisted that he has no option but to "to clean" Aleppo and press on with the offensive.
"You have to keep cleaning this area and to push the terrorists to Turkey ... to go back to where they come from, or to kill them," he said.
Russia and Syria have been widely accused of deliberately targeting schools and hospitals in Aleppo with strikes.
But there have been losses on both sides.
Syrian rebels launched an offensive
last month to break the government's siege on eastern Aleppo, killing dozens of civilians in western Aleppo as they tried to push through regime lines in the east.
On Sunday, fresh clashes broke out in the east, with regime shelling killing at least 11 people in the al-Salehin neighborhood, according to the Aleppo Media Center.
Several other neighborhoods were shelled by ground artillery and heavy machine gunfire from helicopters, the UK-based Syrian Observatory for Human Rights (SOHR) said.