"I had to leave my home early this morning at 6 a.m.," he said. "I took nothing with me, so did hundreds of my neighbors. ... I have nowhere to sleep (because) all the houses are occupied (by soldiers)."
Masri described a scene in which bodies lay in the streets because nobody could evacuate them. He said he messaged CNN from outside a hospital, where he sat on the pavement and used its Internet connection.
"I am hungry and cold and exhausted," Masri said. "Nowhere to sleep. ... I am here with other families, too."
The army has taken all but three neighborhoods, Masri said. An estimated 100,000 civilians are trapped in the eastern section of the city. Meanwhile, the bombing continues.
"I don't know what I am going to do," Masri said.
The Syrian army and its allies have aggressively pushed into the rebel sections of east Aleppo in recent days, said the Syrian government and the Syrian Civil Defence, also called the White Helmets.
"It is going to be a very difficult situation because there is no place for all these fleeing civilians," said Ismail Abdallah, a member of the White Helmets.
"We believe there are a large number of civilian casualties as a result of this intensifying offensive and White Helmets rescue workers are unable to move to reach any of these areas," he said.
Abdallah told CNN the al-Fardous neighborhood has been captured by Syrian government forces, while some areas of resistance remain in parts of the city.
"There is a mass exodus of terrified civilians fleeing from al-Fardous, al-Jaloom, al-Salheine and areas that have been captured today to other areas such as al-Mashhad and Salah Eddiene," he said.
Syrian state television painted a different picture of the day's events, broadcasting video that it said showed residents celebrating in the streets after government forces took control of a neighborhood.
Earlier, Syria's state-run news agency SANA and the Syrian Observatory for Human Rights monitoring group confirmed that the neighborhood of al-Sheikh Saeed in southeastern Aleppo had been captured by the Syrian military.
The neighborhood was taken from rebel forces with the assistance of government-allied groups.
As the situation in Aleppo changes rapidly, CNN will update the map with information from sources on the ground.
Large numbers of civilians are also reportedly fleeing the al-Sukkari neighborhood after intense shelling. The exodus of residents coincided with regime forces advancing.
The SANA report states that army units are "continuing the operations and targeting the remnants of the terrorists who fled away towards" the al-Sukkari and al-Fardous neighborhoods.
In addition, there are reports of civilian casualties as a result of the shelling, according to media organizations with teams in Aleppo.
Failed attempts at peace
The international community has failed to broker a ceasefire for the city, which appears on the brink of falling back into regime control.
UN Secretary General Ban Ki Moon released a statement saying: "The secretary-general is alarmed over reports of atrocities against a large number of civilians, including women and children, in recent hours in Aleppo. While stressing that the United Nations is not able to independently verify these reports, the secretary-general is conveying his grave concern to the relevant parties."
On Monday, Pope Francis reached out to Syrian President Bashar al-Assad, asking him "to ensure that international humanitarian law is fully respected with regard to the protection of the civilians and access to humanitarian aid," according to a statement from the Vatican.
The Pope made the appeal in appointing Cardinal Mario Zenari to the position of Apostolic Nuncio to Syria.
The government controls western Aleppo and its troops have made significant territorial gains in the east since its forces entered the enclave by ground on November 27, backed by continual airstrikes. The government has now taken more than three-quarters of the area.
Rebel groups held eastern Aleppo for more than four years after the Arab Spring uprising, and a Syrian regime siege on the area had essentially cut it off from the outside world, sparking a humanitarian crisis there.
Now civilians are fleeing by the tens of thousands as the relentless airstrikes leave little hope of survival.
An estimated 10,000 people fled over the weekend, civilians told CNN -- a number backed by Russian officials. SANA put the figure at around 20,000.
CNN Senior International Correspondent Fred Pleitgen witnessed thousands of civilians walking through the southern front lines, many gaunt with fatigue and malnutrition, as the children among them cried in fear.
It's difficult to know how many civilians remain trapped in eastern Aleppo, but 100,000 are estimated to still be living in the enclave. Some are beginning to return to neighborhoods retaken by government forces, faced with the daunting task of rebuilding their homes and communities.
CNN's Fred Pleitgen,