At the end of March, Syrian rebels captured the north-western provincial capital of Idlib as government forces fled in the face of a co-ordinated offensive.
The rebels followed up the victory by taking control of the town of Jisr al-Shughour, several villages and the al-Qarmid military base.
They have reportedly now set their sights on the neighbouring Mediterranean coastal province of Latakia, a stronghold of President Bashar al-Assad.
State media have denied that the rebels are gaining ground. But with the regime suffering a string of defeats, and seemingly unable to replace the soldiers and militiamen it is losing, questions are being raised about its resiliency.
Initial reports suggested that Idlib had been seized by al-Qaeda's affiliate in Syria, al-Nusra Front. But in fact the city fell to a rebel coalition known as Jaish al-Fatah (Army of Conquest).
Al-Nusra and the hardline Islamist group Ahrar al-Sham are the coalition's most prominent members, but it also includes several more moderate Islamist groups. All are opposed to Islamic State (IS) and have fought bloody battles with the jihadist group over the past year.
The rebel offensive in Idlib has drawn together some 6,000 fighters, including almost 1,200 from al-Nusra, and brought about unprecedented levels of co-ordination.
But arguably more significant has been the support the coalition has received from regional powers. For the first time, there has been clear co-operation between Turkey, Qatar and Saudi Arabia.
"The regional priorities are changing," says Ibrahim Hamidi, a Syria expert at the pan-Arab newspaper al-Hayat who is originally from Idlib.
Mr Hamidi believes this has prompted the Saudi government to be more tolerant of Syria's Muslim Brotherhood.
Their relationship has been difficult for years, and in 2014 Riyadh designated the Islamist movement a terrorist organisation. Ankara and Doha have meanwhile consistently supported the Brotherhood since the uprising against President Assad began four years ago.
"The change in Saudi Arabia has had an effect on the relationship with Turkey and Qatar," Mr Hamidi says.
By working together, the regional powers appear to have been able to influence the actions of Jaish al-Fatah in Idlib - only the second provincial capital to have been lost by the government, after Raqqa in 2013.
There were fears that the rebels would seek to implement a strict interpretation of Islamic law, or Sharia, but instead they established civil councils to oversee law and order, and public services.
An audio message purportedly by al-Nusra's leader, Abu Mohammed al-Julani, declared that the Idlib would be governed according to Sharia, but Mr Hamidi believes its application will be limited.
"To my knowledge, Saudi Arabia and Qatar put pressure on Julani to ensure al-Nusra's approach would be a moderate one," he says.
"It was clear in Julani's message that he was not going to declare Idlib the capital of an Islamic emirate. The most important thing was that Julani said his group would not be the only one ruling, and that there would be 'Shura' (consultation) and a civil council."
Hadi al Abdalla, a citizen journalist who reported on the battle for Idlib from the frontline, says Western media coverage has mistakenly focused on the presence of al-Nusra.
"For the first time all the groups here are united and they insist that they want civil rule rather than Sharia rule," he explains.
When Jaish al-Fatah captured Idlib, another rebel coalition fighting far to the south took control of the Nassib border crossing with Jordan.
The Southern Front, which includes various groups from the Western-backed Free Syrian Army (FSA) and some Islamist brigades, is now talking of co-ordination with other groups in Deraa province and in the Damascus countryside.
The leader of Jaish al-Islam (Army of Islam), which is backed by Saudi Arabia and operates in the suburbs of the capital, meanwhile visited Turkey last week and was said to have met Turkish officials as well as members of the main opposition alliance, the National Coalition.
Many in the opposition are hoping the changes on the ground in Syria will enable them to increase pressure on the regime.
The UN special envoy, Staffan de Mistura, failed earlier this year to persuade the warring parties to agree to local ceasefires, or "freeze zones", in the northern city of Aleppo.
But now he is scheduled to meet Syrian opposition representatives in Geneva in May, and peace talks with the government might follow if both sides agree.
While there have been clear changes in the approaches of the rebels and their regional backers, it remains to be seen whether they will fundamentally alter the military balance in Syria or lead to a political breakthrough that will finally end the conflict.