Aleppo offensive: Air raids pound Aleppo as US decries 'war crimes'

Air strikes pummelled the shrinking rebel enclave in Aleppo on Saturday as US Secretary of State John Kerry said the Syrian regime's "indiscriminate bombing" amounted to crimes against humanity.

Western powers meeting in Paris called for the resumption of peace talks and for civilians to be allowed to leave Aleppo, where tens of thousands have already fled a fierce regime offensive.

The diplomatic flurry came as a US-backed alliance announced it would launch the second phase of its battle for the Islamic State group's de facto Syrian capital Raqqa further east.

The regime's more than three-week-old assault aimed at retaking all of Aleppo has triggered mounting international outrage.

"The indiscriminate bombing by the regime violates rules of law, or in many cases, crimes against humanity, and war crimes," Kerry said after the talks in Paris, urging Russia to do its "utmost to bring it to a close".

US and Russian officials meanwhile were to gather in Geneva for what Kerry described as a bid to stop the city from "being absolutely, completely, destroyed".

Once the beating heart of Syria's industrial and commercial industries, Aleppo has witnessed some of the most brutal violence of the country's nearly six-year war.

In less than a month, forces loyal to President Bashar al-Assad have overrun around 85 percent of east Aleppo, a rebel stronghold since 2012.

The UN's Syria envoy Staffan de Mistura said the world is watching "the last steps" in the Aleppo battle and evacuating civilians must be a priority.

Air strikes and regime rocket fire battered the last remaining rebel districts Saturday, according to the Syrian Observatory for Human Rights.

An AFP correspondent in west Aleppo could hear the hum of airplanes circling above, coupled with bombardment and machine gunfire on the city's east.

The strikes were so intense that windows in the west rattled and plumes of smoke could be seen rising from several points across the city's skyline.

'Bombing is unreal'

"The bombing is unreal," said Ibrahim Abu al-Leith, spokesman for the White Helmets rescue force inside Aleppo.

Abu al-Leith spoke to AFP from one of the last rebel-controlled zones in Aleppo's southeast, saying he had been forced to move homes because of the intensity of the raids.

"The streets are full of people under the rubble. They are dying because we can't get them out," he said.

According to the Observatory, nine civilians were killed on Saturday in a barrage of rebel rocket fire on government-controlled neighbourhoods.

The fresh attacks brought to 129 people, including 39 children, the number of people killed by rebel fire on government-held west Aleppo since 15 November. 

Another 413 civilians, among them 45 children, have been killed in east Aleppo in the same period. 

With the fighting intensifying after a brief respite, the UN General Assembly demanded an immediate ceasefire and urgent aid deliveries, in a resolution adopted by a strong majority.

But both Moscow and Damascus have rejected talk of a ceasefire without a rebel withdrawal from the city, a demand that opposition groups have refused.

After meeting with opposition representatives on Saturday, French Foreign Minister Jean-Marc Ayrault said the opposition was willing to resume peace talks "without pre-conditions". 

Kerry was not upbeat about the chances of success ahead of the meetings in Paris and Geneva.

"I know people are tired of these meetings, I'm tired of these meetings," he said.

"But what am I supposed to do? Go home and have a nice weekend... while people are dying? Sit there in Washington and do nothing?"

Two thousand more flee

Tens of thousands have fled east Aleppo in recent weeks, with another 2,000 civilians pouring out of the remaining rebel-held districts on Saturday, according to the Observatory. 

State news agency SANA also reported the displacement, but gave a number of 3,000 people and said they had been taken to the temporary shelter in Jibrin, about 10 kilometres (six miles) east of Aleppo.

The UN said Friday it had received reports of rebels blocking some from leaving and of reprisals against residents who asked armed groups to leave.

It has also expressed concern about reports that hundreds of men had gone missing after fleeing to government-held territory.

The fall of east Aleppo would be the biggest blow for the rebels since Syria's conflict broke out in 2011. 

It began as a widespread protest movement against Assad's government but has since evolved into an all-out war that has seen militants like the Islamic State group rise to prominence.

On Saturday, the US-backed Syrian Democratic Forces announced "phase two" of the campaign for the Islamic State group's bastion of Raqqa.

US Defence Secretary Ashton Carter announced that Washington was sending an additional 200 troops to join the 300 it has already deployed in support of the offensive.