Syrian rebels returned to the major northern city of Aleppo after nearly a decade away as part of a sweep which experts believe could eventually lead them to target the capital Damascus.
Fighters opposed to President Bashar al Assad launched what was the first opposition attack on Aleppo since 2016.
Mr Assad has been at war with opposition forces seeking to remove him from power for 13 years.
The rebels claim to have also entered the city of Hama – while independent bodies report a number of other locations have also been taken by the fighters.
Who are the rebels?
The attack on Aleppo has been carried out by a wide variety of Mr Assad’s opponents, including Islamist factions and moderates, which are rumoured to be receiving backing from Turkey.
But it is mostly being led by the jihadi group Hayat Tahrir al Sham (HTS).
A powerful group which was once known as the Nusra Front, HTS is said to have around 30,000 troops.
It has long been designated a terrorist organisation by the US and Russia, but it controls Idlib and has established rule over around four million people.
The al Qaeda-linked group was formed to oppose the Syrian government and was founded by Abu Muhammad al Golani.
But the group was seen as an unwelcome intervention by many in Syria’s opposition, who hoped to keep the fight against Mr Assad’s brutal rule untainted by violent extremism.
HTS took responsibility for deadly bombings and pledged to attack Western forces.
Experts said HTS has been trying to reinvent themselves in recent years, focusing on civilian government in their territory as well as military action.
The group broke ties with al Qaeda in 2016 and Mr Golani has attempted to portray himself as a protector of other religions.
In 2023, he allowed the first Christian mass in years in the city of Idlib.
Middle East analyst Aaron Zelin said that in 2018, the Trump administration suggested it was no longer directly targeting Mr Golani, with other experts saying that while it retained its conservative religious foundations, it had changed its focus to Syria, away from al Qaeda’s global approach.
But, Mr Zelin added that as recently as 2022, members of HTS did shoot at US special forces operating in the country.
The US’s Commission on International Religious Freedom said in a factsheet in 2022 that, despite the group’s “rebrand”, its “Salafi-jihadism… restricts the religious freedom of non-conforming Sunni Muslims and threatens the… safety and existence of [other] religious minority groups”.
Former head of the British Army’s chemical weapons unit, Hamish De Bretton-Gordon, told Sky News it was possible Turkey might be backing the rebels.
“That is something that is rumoured,” he said.
“When you look at HTS, they look (like) pretty professional soldiers. They are well dressed and well equipped.
“A remarkable situation. They took Aleppo city in two days. It was exactly eight years ago – in December 2016 – that Aleppo fell to the Russians and the Syrians after four years of conventional fighting.
“So it is a remarkable change. It is true that HTS are Islamists and a lot of people are concerned of having jihadists taking large swathes of Syria.
“However, a lot of their force is the pre-Syrian army who were the moderates who first in the Arab Spring in 2011 opposed Assad.
“This is Russia’s southern flank. It is giving Putin some really difficult decisions to make.”
What areas have the rebels moved into?
The Syrian Observatory For Human Rights, which monitors the country’s unresolved civil war, said HTS and supporting factions have managed to take control of other major areas.
These include Halfaya, Kafr Zita, Lahaya, Kafr Nabudah, and Morek.
When HTS took Aleppo’s airport at the weekend, it became the first civil airport under its control.
The insurgents have vowed to push all the way into Damascus in the south – but there have been no reports of any incidents or changes to the daily routine in the Syrian capital.
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Syria responds to insurgency with airstrikes on rebel positions
What other nations have interests in Syria?
The surge in fighting has raised the prospect of another violent front reopening in the Middle East.
It comes at a time when US-backed Israel has been fighting Hamas in Gaza and Hezbollah in Lebanon, both Iran-allied groups.
Russia, which is Mr Assad’s main international backer, is also preoccupied with its war in Ukraine.
Previous military intervention by Russia and Iran – alongside support from other groups – had allowed Mr Assad to remain in power and retain 70% of Syria under his control.
The US has about 900 troops in northeast Syria to guard against a resurgence by the Islamic State, which was defeated in Syria and Iraq by 2019, by a US-led coalition that included Kurdish fighters and UK air support.
Both the US and Israel conduct occasional strikes in Syria against Iran-allied forces.
Turkey, which also opposes Kurdish expansion, has forces in Syria alongside its reported influence with the broad alliance of opposition forces.
Syrian opposition fighters drive a seized armoured vehicle. Pic: AP
Why do experts believe this has happened now – and what could develop?
Mr Bretton-Gordon told Sky News: “Since 2020, it’s gone very quiet – it would appear the rebel forces have been preparing, training and re-equipping and looking for an opportunity to try to get back at Assad.
“Remember, when (Assad) took Aleppo in 2016, 300,000 civilians surrendered and a lot of those were tortured and murdered by the regime – so there’s still an awful lot of bad feeling.
“The timing – there is no coincidence there. With Hezbollah much diminished, the Iranian proxy in the region, and also other Iranian militias, it gave the rebels an opportunity.
“With Russia taking so many forces away from Syria to bolster their special military operation in Ukraine, it left Assad exposed and the rebels have really taken advantage of it.
“To the surprise of everybody, [the rebels] have taken huge swathes of Aleppo and Idlib province, and are now heading south to Hama and potentially, although it’s a long way off, to Damascus itself.”
Pic: Reuters
Fighting has ‘potential to be quite consequential’ – expert
Robert Ford, the last-serving US ambassador to Syria, pointed to months of Israeli strikes on Syrian and Hezbollah targets in the area – and to Israel’s ceasefire with Hezbollah in Lebanon – as factors which gave the rebels the opportunity to advance.
Mr Ford said the fighting in Aleppo would become more widely destabilising if it drew Russia and Turkey into direct heavy fighting against each other.
Charles Lister, an analyst with the US-based Middle East Institute, said the fighting “has the potential to be really quite, quite consequential and potentially game-changing” if Syrian government forces prove unable to hold their ground.
Coming after years of few changes in Syrian territory, the new violence risks Islamic State fighters seeing it as an opportunity to advance their interests, experts believe.
The longevity of the unrest in Syria has also seen some 6.8 million Syrians flee the country – a refugee flow which has applied pressure to the political map in Europe.