Opposition forces seeking to overthrow Syria's President Bashar al-Assad launched their biggest offensive in years last week, retaking the northern city of Aleppo and driving government forces out of the region.

It was the first significant development in years in Syria's grinding civil war, which after almost 14 years had largely fallen from the world's headlines. It also raises fresh questions about the opposing sides, who supports them, and what might happen next. Here's what we know:

What happened in Aleppo?

Syrian rebels launched a two-pronged offensive on Aleppo last week, gaining control of the country's second largest city amid little resistance from government troops, according to residents and fighters. 

The insurgents are a coalition of Turkey-backed mainstream secular groups spearheaded by Hayat Tahrir al-Sham (HTS), an Islamist group designated as a terrorist organization by the U.S. and United Nations.

The rebels have since pressed their advance to the south and southwest of Aleppo, capturing territory in Hama province and moving into the countryside around Idlib. 

WATCH | Syrian rebels advance:

Rebel forces advance as Syrian military retaliates with airstrikes

Russian and Syrian jets struck the rebel-held city of Idlib in northern Syria on Sunday, military sources said, as President Bashar al-Assad vowed to crush insurgents who had swept into the city of Aleppo.

Which rebel groups are involved?

HTS, formerly known as the Nusra Front, was al-Qaeda's official wing in the Syrian war but the groups broke ties in 2016. 

Another rebel group — the National Coalition of Syrian Revolution and Opposition Forces, the internationally recognized Syrian opposition — launched a separate offensive from north of Idlib. It represents anti-Assad groups including the Turkey-backed Syrian National Army or Free Syrian Army.

Why now?

The assault followed the recent ceasefire between Israel and Hezbollah, ending more than a year of fighting that started shortly after Israel went to war with Hamas in Gaza.

Hadi al-Bahra, president of the National Coalition, says rebels began preparing to seize Aleppo a year ago but were delayed by the Israel-Hamas war.

"The moment there was a ceasefire in Lebanon, they found that opportunity... to start," he said. 

Earlier this year, Israeli airstrikes in Aleppo hit both Hezbollah weapons depots and Syrian government forces, among other targets, according to an independent monitoring group. Israel rarely acknowledges strikes against Aleppo or other government-held areas of Syria.

Iran which, like Hezbollah, supports the regime, has also been weakened by recent Israeli airstrikes.

Russia — Assad's main international backer — is meanwhile preoccupied with its war in Ukraine. 

WATCH | Russian airstrike kills at least 12:

Inside an Aleppo hospital after a deadly Russian military airstrike

A Russian airstrike at the Aleppo University Hospital in Syria killed at least 12 people and injured more than 20 others. The bombardment, in support of Syrian government forces, followed insurgents seizing control of Aleppo, the country's largest city.

Why Aleppo is significant

Aleppo, an ancient centre of commerce and culture in the Middle East, was home to 2.3 million people before the war. Rebels seized the east side of the embattled city in 2012, and it became the proudest symbol of the advance of armed opposition factions, but changed hands again when a brutal air campaign by Russia helped al-Assad retake the city.

Intervention by Russia, Iran, Hezbollah and other groups did much to keep Assad in power, who now has 70 per cent of Syria under his control. The rest is held by a range of opposition forces and foreign troops. 

This latest change "has the potential to be really quite, quite consequential and potentially game-changing," if Syrian government forces can't hold their ground, said Charles Lister, a longtime Syria analyst with the U.S.-based Middle East Institute.

Which countries are involved?

Syria is split into three regions: each controlled by either the Assad regime, opposition forces or the Syrian Democratic Forces, a Kurdish-led coalition of U.S.-backed ethnic militias and rebel groups opposed by Turkey.

Russia and Iran have sway over government-held areas, the single biggest chunk of Syria. The U.S. has forces in the northeast and east, backing the Kurdish-led Syrian Democratic Forces. Turkey has troops in the rebel-held northwest. 

WATCH | Syrian army redeploys:

Syrian rebels capture most of Aleppo in 1st major offensive since 2020

The Syrian army said on Saturday dozens of its soldiers had been killed in a major attack by rebels who swept into the city of Aleppo, forcing the army to redeploy in the biggest challenge to President Bashar al-Assad in years.

Hezbollah, the regime's third main supporter, does not currently intend to send fighters to northern Syria to support Assad, according to sources who spoke to Reuters.

Turkish forces have previously attacked northeast Syria, wanting to eliminate one of its main concerns on its borders: Kurdish-led groups.

Iran has pledged to aid the Syrian government and on Monday hundreds of fighters from Tehran-backed Iraqi militias crossed into Syria to help fight the rebels, Syrian and Iraqi sources said.

What happens now?

Syria's government has scrambled its forces to push back the offensive, with help from Russia. Both the government and Russia have been rushing reinforcements and striking rebel-held areas as they attempt to stall the opposition's momentum. 

Airstrikes pummelled Idlib in northwestern Syria on Monday, killed at least a dozen civilians including children, according to the White Helmets,  a volunteer rescue group also known as Syrian Civil Defence.

At least 44 civilians, including 12 children and seven women, were killed in northwest Syria from Tuesday to Saturday, according to the United Nations Office for the Co-ordination of Humanitarian Affairs (OCHA).

OCHA added that over 48,500 people had been displaced, with key infrastructure — including hospitals, schools and water stations — damaged in the escalating hostilities.

The United Nations has described the situation in Aleppo as "volatile and unpredictable."

"The latest developments pose severe risks to civilians and have serious implications for regional and international peace and security," said UN Syria envoy Geir O. Pedersen in a statement.