Syrian rebels renewed their bid to break a government siege on eastern Aleppo on Thursday, shelling regime-held parts of the city, hours before a brief Russian-declared ceasefire was due to begin.

State media said at least 12 people had been killed in rebel rocket and gunfire on western neighbourhoods, after an assault that began with a double car bomb attack.

Meanwhile, the spokesman for a Syrian Kurdish-Arab alliance known as the Syrian Democratic Forces said the group would lead the fight to recapture the Islamic State group stronghold of Raqa.

But he stressed that they rejected any role for Turkey, which in August began a military operation inside northern Syria targeting both IS and Kurdish fighters.

In Aleppo, rebels were engaged in fierce fighting with government forces in several districts on the western outskirts, the Syrian Observatory for Human Rights monitor said.

The Britain-based group said the "most intense fighting" was on the edges of the Halab al-Jadida neighbourhood.

"If they take Halab al-Jadida, they could advance towards the military complexes," Observatory chief Rami Abdel Rahman told AFP.

The clashes on Aleppo's western outskirts could be heard in the city's eastern districts, an AFP correspondent there said.

Rebels including former Al-Qaeda affiliate the Fateh al-Sham Front began an assault on west Aleppo on Friday in a bid to break a three-month government siege.

No aid has reached eastern Aleppo city's more than 250,000 residents since early July, and there are reports of shortages and price hikes.

- Civilians killed -
The Observatory said government forces were carrying out air strikes in areas where clashes were underway, but that the rebel-held neighbourhoods in the east of the city were largely quiet.

Syrian state television said at least 12 people had been killed and over 200 wounded in rebel fire on Thursday.

The Observatory put the toll at 11 dead, saying four children were among them, bringing to 65 the total number of civilians killed in rebel fire since Friday, including 23 children.

State media also said eight people were being treated for suffocation after "terrorists fired poisonous gases" at the Minyan village west of Aleppo.

Once Syria's economic powerhouse, Aleppo has been ravaged by the conflict that began with anti-government protests in March 2011.

It has been divided since mid-2012 between government control in the west and rebel control in the east, and in September the army launched an operation to recapture the city.

That offensive, backed by ally Russia, killed hundreds of civilians and hit infrastructure including the east's few remaining health facilities.

It provoked international recriminations, particularly against Russia, with the EU accusing it of actions that "may amount to war crimes".

Moscow has rejected the criticism and in late October declared a unilateral three-day ceasefire, during which it urged civilians and rebels to leave east Aleppo.

But only a handful did, with Russia accusing rebels of preventing people from leaving, and a UN plan to evacuate wounded people was shelved over security concerns.

- SDF to 'lead' Raqa fight -
Moscow said Wednesday it would implement a new, 10-hour "humanitarian pause" for Aleppo on Friday, prompting the UN to warn that "humanitarian operations cannot be contingent on political or military initiatives".

The pause was meant to "prevent senseless casualties", the chief of Russia's General Staff Valery Gerasimov said.

More than 300,000 people have been killed in Syria since the conflict began with anti-government protests that spiralled into a multi-front war involving regime, rebels, Kurds and jihadists like IS.

The Kurdish-Arab Syrian Democratic Forces alliance has been fighting to oust IS from areas across northern Syria with air support from the US-led coalition against the jihadist group.

The alliance said Thursday it would fight to recapture the IS stronghold of Raqa in a battle that could start in coming weeks.

"We will see a campaign led by the Syrian Democratic Forces to liberate Raqa city," SDF spokesman Talal Sello said.

"We are ready. We have the sufficient numbers for this campaign and we will start it soon," Sello told AFP, without specifying a timetable.

But he insisted that the fight would not include Turkey, which views the YPG as linked to the outlawed Kurdistan Workers' Party (PKK), considered by Ankara as a "terrorist" group.

"The topic of Turkey's participation was settled with the (US-led anti-IS) coalition definitively. No Turkish participation," he said.