* Rebels summon all fighters against government offensive
* Aleppo split largely in half through much of civil war
* Assad trying to solidify foothold in rebel-dominated north
By Khaled Yacoub Oweis
AMMAN, Nov 12 (Reuters) - Syrian government forces backed by foreign Shi'ite Muslim militia advanced on rebels in the northern city of Aleppo on Tuesday, bent on recapturing districts from opposition brigades weakened by infighting, activists said.
They said that rebels saw the threat of President Bashar al-Assad wresting back Aleppo, Syria's former commercial hub and once most populous city, as so grave that Islamist brigades, including an al Qaeda affiliate, had declared an emergency and summoned all fighters to head to the fronts.
After 2-1/2 years of conflict, which started when Assad's forces fired on pro-democracy demonstrators and escalated into a full-blown civil war, the fighting has settled into a rough stalemate in which scores of people are killed every day.
Aleppo has been divided roughly in half by the warring parties for much of the conflict but the government is determined to reassert total control to solidify a foothold in the north where rebel supplies stream in from Turkey.
The rebel groups' joint declaration said government forces backed by Lebanese Hezbollah guerrillas, Iranian Revolutionary Guards and the Iraqi Abu al-Fadl Abbas militia had launched "a fierce offensive to reoccupy" Aleppo.
Dozens of men from both sides have been killed in the last few days in embattled northern and eastern areas of the city. The fighting has also involved the Islamic State in Iraq and the Levant, another al Qaeda branch comprised mainly of foreign fighters that has been gaining ground in the north.
Opposition sources said Iraqi Shi'ite and Hezbollah fighters based near Damascus had moved north to support the offensive on Aleppo. Hezbollah and Iran do not comment on the scale of their military involvement in Syria.
Activist Mohammad Nour of the Sham News Network opposition monitoring group said large neighbourhoods in Aleppo such as the eastern district of Hananu which have been largely rebel-controlled for more than a year were now looking vulnerable.
"Regime forces aided by Hezbollah, the Iraqis and the Iranians have launched a pincer movement from the north and the east and are closing in on major neighbourhoods," he said.
"Infighting has undermined Aleppo's defences," he said, referring to clashes in the past two months inside the city and in its northern rural environs between al Qaeda affiliates and units belonging to the Western-backed rebel Supreme Military Council, whose command is based in Turkey.
Islamist units have also fought among themselves over land.
The United States and European allies hope a proposed Syria peace conference in Geneva will yield an interim government that can help end the bloodshed raging since 2011.
Activists said Assad's forces backed by tanks had taken two highrise buildings in the northern Ashrafieh and Bani Zeid districts, and advanced into the two neighbourhoods after close-quarter street fighting.
The Tawhid Brigades sent reinforcements to the eastern al-Naqqarin district after Assad's forces and their militia allies penetrated the area, the opposition sources said.
Rebels have held most of eastern Aleppo and several districts in the west and centre since fighters based in the rural hinterland and in impoverished outlying districts stormed the city in July last year.
Tareq Abdelhamid, an activist well-connected with different brigades in Aleppo, said: "Luckily the regime seems to be underestimating how much the (internal) divisions have sapped rebel strength and has been overcautious in its advance."
Government forces recaptured at the start of November the town of Safira southeast of Aleppo on a main supply route to Hama and, with Hezbollah help, an army base near Aleppo airport after the compound changed hands several times.
Assad is from the minority Alawite sect, an offshoot of Shi'ite Islam that has controlled Syria since the 1960s.
More than 100,000 people have been killed in Syria's increasingly sectarian conflict, pitting Alawites and Shi'ite supporters backed by Iran against mainly Sunni rebels who are supported by Saudi Arabia and Qatar. (Editing by Mark Heinrich)