Regime forces kill 60 after storming several areas of eastern Aleppo

60 civilians have been killed after the Syrian regime army stormed several areas of eastern Aleppo, Syrian Observatory for Human Rights said on Monday.

The regime forces made new advances Monday after taking the Sheikh Saeed district, leaving civilians and opposition fighters trapped in a tiny part of the city.

Meanwhile, regime and Russian warplanes carried out several raids on opposition-held neighborhoods in eastern Aleppo, Najib Ansari said.

He said the fierce bombardment on the war-battered city has forced thousands of civilians to flee to regime-controlled areas in Aleppo.

Ansari warned that the humanitarian situation in besieged neighborhoods in eastern Aleppo is on the verge of collapse.

"Fighters from [Lebanon's] Hezbollah and [Iraq's] Al-Nujaba militias are preventing civilians from leaving the city," one local source who preferred anonymity said.

UN High Commissioner for Human Rights spokesman, Rupert Colville, said Friday hundreds of men remain missing more than a week after crossing into Syrian regime-held areas of Aleppo.

Assad regime's first major advance since Russia's intervention in Sep. 2015 was cutting off opposition's supply route from Turkey and lifting the siege on Zahraa and Nubl towns in early Feb. 2016.

The PYD/YPG forces in Afrin also moved eastwards to opposition-held territory, capturing Menagh Air Base and the town of Tal Rifaat while pressuring Azaz, preventing an opposition counter attack.

In late July, regime forces attacked the Castello Road, the only remaining supply route of the opposition to east Aleppo, effectively laying siege on some 300,000 civilians.

Opposition fighters launched an attack to Ramouseh district and Artillery College in early August, managing to open a new supply route after fierce clashes and cut off regime's supply routes.

This route failed to function properly amid regime's air and artillery strikes, and the regime launched a new attack in early September, laying siege on east Aleppo once again.

The regime launched an attack in northern Aleppo in mid-October, capturing the Handarat refugee camp and several neighborhoods in northern parts of the city.

Opposition launched a large scale attack on w. Aleppo in late October, failing to make any gains amid staunch resistance by the regime, which launched its counter-offensive on e. Aleppo on Nov. 26.

Opposition fighters suffered huge setbacks against regime forces on Nov. 27, retreating in most parts of northeastern Aleppo. Regime forces managed to cut off and isolate several neighborhoods.

On Nov.28, civilians in captured areas fled to PYD-held Sheikh Maqsood neighborhood and southwards to opposition controlled parts, along with fighters moving south to reinforce the area.

On Dec. 1, regime forces launched a new attack to capture the parts of Aleppo's old city controlled by the opposition.

By Dec. 4, regime forces managed to advance in northeastern axis, capturing Karm al-Myassar, Karm al-Qatirji and Karm al-Tahan districts from opposition fighters.

Regime forces managed to drive opposition fighters out of the neighborhoods located east and south of the Aleppo citadel by Dec. 7.

By Dec. 11, regime forces advanced into Sheikh Saeed district as opposition groups started retreating west of the Kouwalk River amid relentless regime bombing an shelling.

Regime forces captured Bustan al-Qasr and al-Kallasah districts east of Kouwalk River, effectively controlling over 90 percent of Aleppo and leaving some 150,000 people trapped under bombardment.

Since mid-November, more than 990 civilians have been killed - and some 2,500 injured - by regime attacks on opposition-held parts of Aleppo, according to local civil defense officials.

The fierce bombardments have forced most of the city's hospitals to halt operations, while most academic activities have been indefinitely suspended.

The recent escalation comes amid attempts by the Russia-backed Assad regime to reestablish control over parts of Aleppo captured four years ago by armed opposition groups.

Syria has been locked in a devastating civil war since early 2011, when the Assad regime cracked down on pro-democracy protests - which had erupted as part of the "Arab Spring" uprisings - with unexpected ferocity.

Since then, hundreds of thousands of people are believed to have been killed and millions more displaced by the conflict.