What do we know about the offensive, the countries involved and the prospects for Syria's future?

- Who launched the attack? -

Rebel group Hayat Tahrir al-Sham (HTS), led by Syria's former Al-Qaeda affiliate, and allied factions launched their offensive against government forces in Aleppo province on Wednesday.

The fighting has killed hundreds, though the war monitor the Syrian Observatory for Human Rights and eyewitnesses say government troops withdrew from many positions without any serious resistance.

The rebels, based in the northwest in the opposition bastion of Idlib, have seized the city of Aleppo from government control, though some northern neighbourhoods remain Kurdish-held.

The city's east, once a stronghold for opposition factions, was recaptured by the government in 2016 in a devastating siege aided by Russian air support.

Jerome Drevon from the International Crisis Group said the rebel offensive initially aimed to "test... the regime's defensive lines" but "the regime was weaker than they thought".

Rebel control of Aleppo shows "Iran and Russia either couldn't or didn't want to really protect the regime at an essential point, which is the economic capital (Aleppo)", he told AFP.

Fighters have seized dozens of cities and towns in Idlib, Hama and Aleppo provinces, the Observatory said, reporting heavy clashes as rebels advanced on Tuesday towards the city of Hama amid Russian and Syrian air strikes.

In parallel, Ankara-backed Syrian opposition factions based along the country's border with Turkey have launched their own fight in recent days around the city of Aleppo.

Turkish forces and their proxies have controlled swathes of territory in northern Syria since 2016 when Ankara began successive ground operations to expel Kurdish fighters it says are linked to a group waging a decades-long insurgency against the Turkish state.

To Aleppo's east, the pro-Turkey factions have been fighting government forces, while to the north they have been fighting United States-backed Kurdish fighters.

- What countries are involved? -

President Bashar al-Assad said on Saturday that Syria, "with the help of its allies and friends", is capable of defeating "terrorists and their backers" -- his first remarks since the offensive began.

Hans-Jakob Schindler, director of the think tank the Counter Extremism Project (CEP), said now was the ideal time for the rebels to attack.

"The Assad regime is truly not very strong," he told AFP.

Its major backers, Russia and Iran along with Lebanon's Hezbollah, "all have other issues to deal with right now", he added.

Assad has long counted on military, economic and diplomatic support from Tehran and Moscow, which is tied up with its conflict in Ukraine, while Lebanon's Iranian-backed Hezbollah has taken a beating in its recent war with Israel.

The Russian military said on Sunday it was helping the Syrian army against the rebel forces, while Iran said on Monday it planned to keep military advisers in the country.

Also on Monday, Russian President Vladimir Putin and Iranian counterpart Masoud Pezeshkian "expressed unconditional support" for Syria's government -- and "emphasised the importance" of coordinating with Turkey, the Kremlin said.

Turkish Foreign Minister Hakan Fidan said "Damascus must reconcile with its own people and the legitimate opposition".

Some experts say the rebel offensive could not have happened without a green light from opposition-backer Ankara.

- What prospects for the future? -

Aron Lund of the Century International think tank said "Aleppo seems to be lost for the regime."

"Unless they manage to mount a counteroffensive soon, or unless Russia and Iran send much more support, I don't think the government will get it back," he told AFP.

"A government without Aleppo is not really a functional government of Syria," he added.

Tammy Palacios, senior analyst at the New Lines Institute, said "Assad does not likely have the resources to take back Aleppo".

Doing so would require support from Iran's Revolutionary Guards and Russian air power "and would not likely extend to retaking the towns surrounding Aleppo", she added.

Syria has not experienced similar levels of fighting for years.

A 2020 Turkish- and Russian-brokered truce in the Idlib region had been largely holding despite repeated violations.

But a United Nations-led political process has stalled, and no sustainable resolution to the conflict seems to be on the horizon.

Fabrice Balanche, a lecturer at France's Lumiere Lyon 2 university, said it was unlikely there would be a "reunification" of Syria.

"The existing fragmentation will settle in over time," he predicted.

"The regime can maintain itself in the Alawite coastal region, particularly thanks to Russia's presence in Homs and Damascus", he said, referring to Assad's minority community.

But the northwest will stay "under the tutelage of HTS and pro-Turkey" groups, while Kurdish forces will dominate the northeast "if the American presence remains" and prevents a Turkish offensive, he told AFP.

A UN-recognised government could co-exist alongside "autonomous entities... relying on a strong ethno-sectarian identity", he added.

burs-lar/lg/dcp