BEIRUT (Reuters) - Syrian insurgents stepped up a week-long offensive on government-held areas in the city of Aleppo on Thursday, detonating three car bombs and firing shells which killed at least a dozen civilians, state media and a monitor said.

Rebel groups detonated three large car bombs near pro-government forces on the western edge of Aleppo, the Syrian Observatory for Human Rights and rebels said, as they tried to revitalize an offensive which has made little progress since taking most of Dahiyet al-Assad suburb on Friday.

A senior medic in government-held Aleppo told Reuters eight people had been brought into hospitals suffering from breathing difficulties thought to be from a poisonous gas attack. This could not be independently confirmed.

Insurgent groups, including both Free Syrian Army factions and jihadists, are seeking to break a government siege on rebel-held east Aleppo which has been almost continuously in place since July. They want to seize government-held areas of Aleppo in order to link the city’s east with rebel-held rural areas west of Aleppo.

The city has been divided for years between the government-held western sector and rebel-held east, where the army launched a major Russian-backed offensive in September that medics say has killed hundreds.

REBEL SHELLING

United Nations aid has been unable to reach the besieged area since that time and the local medical infrastructure has been brought to its knees by a lack of supplies, a shortage of staff and air strikes on medical facilities.

Syrian state media said on Thursday 12 people were killed and around 200 injured by the rebel shelling on government-held western Aleppo. The Syrian Observatory for Human Rights said at least 15 civilians, including four children, had been killed in west Aleppo and around 120 injured.

Zaher Hajo, the head of forensic medicine in government-held Aleppo who reported the eight patients suffering respiratory difficulties, said the medical authorities could not determine the nature of the gas believed to have been used.

All parties in the multifaceted Syrian conflict have accused each other of using chemical weapons during more than six years of conflict.

On Sunday Syrian state media said militants fired poison gas at a government-held area of Aleppo, causing 35 people to choke, a report that a rebel official denied.

An international inquiry by the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW) has found Syrian government forces responsible for three toxic gas attacks and has said Islamic State militants have used sulfur mustard gas.

Russia, which has been supporting the Syrian air force in a heavy bombing campaign on Syria for more than a year, says it has not carried out air strikes on eastern Aleppo for more than two weeks.

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Russia and the Syrian army on Wednesday told rebels to leave Aleppo by Friday evening through safe corridors, signaling an extended moratorium on air strikes.