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.
"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.
Late last month, Iran-backed Iraqi Shia militia of Harakat al-Nujaba released a video showing its leader Akram al-Kaabi arriving in Aleppo via the airport, and then meeting his militia members fighting alongside Assad forces and Lebanon's Hezbollah.
The militia members receive the leader with poetry before a song begins over the video in which Aleppo is described as Shia and is compared with the Iraqi city of Karbala, a holy site for Shia Muslims.
The vast majority of Iraqi militias fighting in Syria are Shia, but the Muslim sect is believed to make up only 1 percent of Syria's population.
Phillip Smyth, a researcher at the University of Maryland on Shia armed groups, told Al Jazeera that there was a "major effort" now by Iranian-backed forces operating in Aleppo to show "that they are leading the show and to demonstrate that they are a massive presence on the ground".
"Iraqi Shia militias have been operating on the ground since 2013, but Harakat al-Nujuba was the first to announce that it was operating in Aleppo. They were quite open and and quite glib about their presence there - which is in contrast to the often repeated rhetoric that they are in Syria to protect the Sayyidah Zaynab site in Damascus."
Sayyida Zeinab is one of the most important pilgrimage sites for Shia Muslims.
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Once Syria's economic powerhouse, Aleppo has been hit hard by the war, which has killed more than 300,000 people since it began with anti-government protests in March 2011.
More than 275,000 people remain in the city's rebel-held eastern part, which has been under near-continuous siege since mid-July and has been targeted relentlessly since the government launched its offensive on September 22.
The rebels have responded by intensifying their rocket fire on western Aleppo, where four people were killed in the Midan neighbourhood on Friday, Syrian state television reported.
On Thursday, rebel fire killed at least 11 people in the Al-Jamaliyeh neighbourhood, the Syrian Observatory said.
The Syrian army said earlier this week that it would rein in its bombardment of the east, and the observatory said that there had been a reduction of the bombing that has killed hundreds and destroyed the largest hospital in the rebel-held sector.
But the army has pressed on with its ground offensive, with Assad saying his forces had "no option" but to expel rebels from the city unless they agreed a deal with the government.
In an interview with Danish broadcaster TV2 aired on Thursday, Assad said Aleppo's "best option" would be a reconciliation deal like those the government has negotiated with rebels elsewhere.
Opposition forces say those deals have been forced upon them by a government strategy of "surrender or starve."
Assad said that without an agreement, he would "continue the fight with the rebels till they leave Aleppo ... There's no other option".
State television reported on Friday that people were leaving east Aleppo through humanitarian corridors, but gave no further details and showed no images.
Rescue workers with the White Helmets told Al Jazeera that they saw no sign of people leaving, and residents have previously said they feared entering government-controlled territory.
The assault on Aleppo, which began after the collapse of a truce deal negotiated by Russia and the United States, has prompted international condemnation.
On Thursday, de Mistura warned of the operation's consequences.
"In maximum two months, two and a half months, the city of eastern Aleppo may be totally destroyed," he said.
He urged fighters from the former al-Nusra Front - now known as Jabhat Fateh al-Sham after splitting with al-Qaeda - to leave the city under a deal to halt the government's assault.
"If you decide to leave with dignity ... I am personally ready to physically accompany you," de Mistura said.
Just hours after his proposal, Jabhat Fateh al-Sham said on Twitter that it would not withdraw its fighters from eastern Aleppo.
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Hossam al-Shafai said the group is "determined to break the siege" on the city's rebel-neighbourhoods.
The UN Security Council is to hold an emergency meeting later on Friday at the request of Russia to receive a briefing from de Mistura.
Council members are also discussing a French-drafted resolution calling for a ceasefire in Aleppo.
After holding talks in Moscow on the proposal, Jean-Marc Ayrault, France's foreign minister, is to visit Washington later on Friday to discuss the text, which also calls for a halt to all flights over the city.
Francois Delattre, France's UN ambassador, said on Thursday "we have a strong determination to go to a vote" on the draft.
With reporting by Dylan Collins @collinsdyl
Source: Al Jazeera News And Agencies