Syrian rebel factions fought each other in besieged eastern Aleppo on Thursday, officials from two of the groups and a war monitor said, potentially undermining their efforts to fend off a major Russian-backed offensive.

Rebel groups have been plagued by disunity and infighting throughout the 5 1/2-year-old conflict, for ideological reasons, over tactical differences or in disputes over territory.

Fighters of Jabhat Fateh al-Sham, Nour al-Din al-Zinki and Abu Amara attempted to seize positions and weapons from Fastaqim, one of its officials said. Fateh al-Sham is a jihadist group. Zinki and Fastaqim fight under the Free Syrian Army (FSA) banner.

An official from the Zinki group's politburo said the clashes had finished and there were efforts under way to resolve their dispute.

The clashes took place in the al-Ansari district and the neighboring Salah al-Din district, both of which are close to the front line with government-held areas, said the Syrian Observatory for Human Rights, a Britain-based war monitor.

The Observatory said the Islamist groups had succeeded in taking most positions and weapons from Fastaqim and detaining its fighters, but the group's official said its situation was not so bad.

Both Islamist and FSA groups are taking part in an offensive against government-held western Aleppo that began last week, which is partly aimed at breaking the siege on the rebel-held east but has made little progress after some initial gains.

Syria's army, backed by Shi'ite militias and Russia's air force, succeeded this summer in surrounding eastern districts of Aleppo, besieging the Sunni rebel groups who seek to overthrow President Bashar al-Assad as well as around 250,000 civilians.

The army and Russia on Wednesday told rebels they should leave eastern Aleppo by Friday evening, signaling their temporary halt on air strikes in the city, which began on Oct 18, would extend until then.

Damascus has told rebels and residents of eastern Aleppo that they can have safe passage out of the city, either to other areas held by the opposition or to resettle in government areas, but rebels have rejected that and few people have left.

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

There were clashes between rebel groups and pro-government forces around Dahiyet al-Assad and near the 1070 Apartment Blocks, a district in southwest Aleppo that has been the scene of continued fighting since the summer.

Zinki, which has received foreign support funneled through Turkey, recently joined a joint operations room run by an alliance of Islamist factions known as Jaish al-Fateh. It includes factions like Fateh al-Sham, formerly loyal to al Qaeda and known as the Nusra Front.