Rebels in eastern Aleppo are fighting back against a major government push [Reuters]
Syrian government troops are advancing gradually against opposition fighters inside Aleppo, even as the US secretary of state calls for Russia and Syria to face a war-crimes investigation for alleged attacks on Syrian civilians.
John Kerry spoke on Friday in advance of discussions on a draft UN Security Council resolution that would call for an end to the Russian-backed onslaught on Syria's second city.
The two-week assault by Syrian President Bashar al-Assad's forces has led to a global outcry after air strikes on hospitals and a UN aid convoy.
"These are acts that beg for an appropriate investigation of war crimes, and those who commit these would and should be held accountable for these actions," Kerry said in Washington, DC.
UN envoy says eastern Aleppo faces "total destruction"
"They are beyond the accidental now … way beyond … years beyond the accidental. This is a targeted strategy to terrorise civilians and to kill anybody and everybody who is in the way of their military objectives."
The assault on rebel-held districts of Aleppo has elicited a warning from Staffan de Mistura, the UN envoy, that eastern Aleppo could be "totally destroyed" by the year's end.
Fierce fighting on Friday rocked several districts of the city, which has been divided between government control in the west and rebel control in the east since 2012.
Government forces captured a hilltop in the Sheikh Saeed district in the south of Aleppo, but the rebels retook other parts of the neighbourhood previously captured by the government, according to the Syrian Observatory for Human Rights and rescue workers on the ground.
The UK-based monitoring group also reported clashes in the Salaheddin, Bustan al-Basha and Suleiman al-Halabi neighbourhoods on the city's frontline.
"This morning regime forces tried to take over several points in the Skeikh Saeed area, but rebel forces managed to push them out, taking more than 10 fighters prisoners, all of them from Iraqi Shia militias," Ibrahim Abu Leith, spokesman for the Syria Civil Defence in Aleppo, told Al Jazeera.
Also known as the White Helmets, the volunteer rescue group operates in rebel-held areas across Syria.
Iraqi militia presence
For their part, rebels in Aleppo said on Friday that they had killed at least 23 pro-government fighters, injured dozens and captured at least six Iraqi Shia militia members deployed on behalf of the Assad government.
Thousands of Iraqi Shia militia members and members of Lebanon's Hezbollah are fighting alongside government forces in Aleppo against primarily Sunni rebel forces.
More than 1,000 Iraqi Shia fighters have travelled to Aleppo from Iraq since early September, joining the ranks of another 4,000 already on the ground in the area, militia leaders and Syrian rebels told the Wall Street Journal earlier this week.