Rebels reported signs of large numbers of Shiite Muslim fighters flowing in from Iraq to help Assad end the civil war that has killed at least 80,000 people and forced 1.6 million Syrians to flee abroad.
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The move to a northern front comes as Syria's war is increasingly infecting its neighbors – Iraq, Lebanon, Jordan and Israel – and widening a regional sectarian faultline between Sunni and Shiite Muslims.
For the first time since the start of the uprising in March 2011, an Israeli minister suggested on Monday that Assad might prevail in the war, thanks in large part to support from Shiite Iran and the Lebanese guerrilla group Hezbollah.
However, efforts to dislodge rebels in Aleppo will be a much tougher proposition than last week's capture of the town of Qusair, with military analysts predicting that the conflict will probably drag on for months or years as Assad's many foes are likely to be galvanized by recent rebel reversals.
Alarmed by Assad's swift advances and hoping to turn the tide, Washington might decide later this week on whether to start arming the rebels, a US official said.
Assad's army is preparing to lift sieges on areas close to Aleppo before turning its sights on the country's second city, according to the semi-official Syrian al-Watan daily
"Any battle in Aleppo will be huge and most certainly prolonged," said Charles Lister, an analyst at IHS Jane's Terrorism and Insurgency Centre.
"You have large numbers of rebels in several areas of the city. There will have to be a lot of very close combat fighting that always takes a lot of time and leaves many casualties."
Rebel brigades poured into Aleppo last July and have more than half the great merchant city under their control. The front lines are largely stable and a growing number of radicalized, Islamist foreign fighters have joined rebel ranks.
Opposition activists and military sources said the army was airlifting troops to Aleppo airport and to the Kurdish area of Ifrin behind rebel lines, as well as reinforcing two rural Shiite Muslim enclaves, Zahra and Nubbul, north of the city.
"The regime appears to be making a pincer movement to try and regain the major cities across the north and east of Syria ahead of the Geneva conference," said Abu Taha, a northern rebel commander, referring to proposed international peace talks.
The United States and Russia hope to hold the conference in Switzerland next month, but Britain has warned that Assad's recent success might make him reluctant to offer the sort of compromises believed necessary to end the bloodshed.
After appearing to seize the initiative in 2012, the rebels have suffered a series of setbacks this year, with Assad's demoralized forces significantly bolstered by the arrival of well-trained fighters from the Shiite Muslim group, Hezbollah.
Rebels said these guerrillas had played a determining role in the emphatic victory last week in Qusair, which controls vital supply routes across Syria and into Lebanon.
A security source in Lebanon said Hezbollah would continue to assist Assad, but unlike the battle for Qusair, which lies close to its home turf, it might not dispatch its troops north to Aleppo, preferring instead to offer training.
Looking to relieve the growing pressure on Aleppo, rebels attacked two major military compounds on Monday in northern Syria – on the outskirts of the city of Raqqa and the Minnig airport in the adjacent province of Aleppo.
"The rebels have raised pressure ... in the last two days to pre-empt any attack on Aleppo," said Abdelrazzaq Shlas, a member of the opposition administrative council for the province.
Activists said the army had retaliated by bombing Raqqa, killing at least 20 civilians and fighters.
"There is a big loss of lives, but the aim is to deflate the morale boost that the regime received after Qusair and not allow it to go to Geneva as a victor," Shlas said.
But in a worrying development for the rebels, Shlas said there were reports of militiamen loyal to Iraqi Shiite Cleric Moqtada al-Sadr streaming into Syria to bolster Assad's forces.
Their arrival would underline the increasingly regionalized nature of the war following Hezbollah's entry into the fray.
Lister, who monitors Sunni Muslim Jihadist forums, said it seemed a growing number of Sunni men appeared ready to take up arms in Syria with the mainly Sunni rebel forces.
"If you believe what you read in the forums, then there are a lot of people heading to Syria to take up the fight," he said, adding that there were also a growing number of death notices for foreign fighters appearing on the web, including six in one day last week.
Assad may prevail in Syria warIsrael, which shares a tense border with Syria, has regularly predicted the fall of Assad. But on Monday, Intelligence Minister Yuval Steinitz offered a very different view.
Speaking to foreign reporters in Jerusalem, he said Assad's government "might not just survive but even regain territories."
Though the assessment was quickly disavowed by others in Prime Minister Benjamin Netanyahu's coalition government, it reflected the difficulties facing Israel and Western countries in predicting Syria's destiny and weighing intervention.
The minister was asked at a briefing with foreign journalists whether recent successes by Assad's forces against outgunned rebels might herald victory for the Syrian leader.
"I always thought that it might be the case that at the end of the day Assad, with a very strong Iranian and Hezbollah backing, might gain the upper hand," Steinitz said.
"And I think that this is possible and I thought that this is possible already a long time ago."
He declined to comment further on a possible Assad victory, citing Israel's policy of not meddling publicly in Syria.
Netanyahu spokesman Mark Regev said Steinitz was speaking for himself and that the Israeli government did not have a formal position on Assad's prospects.
Alluding to Israel's superior military might, Steinitz added: "It is in his (Assad's) interest not to provoke us so that we get involved."
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