Islamic fighters led by al Qaida's branch in Syria have seized almost full control of the north-western city of Idlib, taking over major roundabouts and government buildings in a powerful blow to President Bashar Assad whose forces rapidly collapsed after four days of heavy fighting, opposition activists and the extremist group said.
Idlib, a major urban centre with a population of about 165,000 people, is the second provincial capital to fall into opposition hands after Raqqa, now a stronghold of the Islamic State (IS) group.
Its capture by the Nusra Front underscores the growing power of extremist groups in Syria who now control about half the country.
Opposition fighters including Nusra have controlled the countryside and towns across Idlib province since 2012, but Assad's forces have managed to maintain their grip on Idlib city, near the border with Turkey, throughout the conflict.
Today Islamic fighters jubilantly swept in, taking over key buildings and tearing down posters of Assad. Videos posted online by activists and the Nusra Front showed a group of heavily armed fighters kneeling down in prayer in the city's sprawling Hanana square as others fired their guns in celebration.
"Allahu Akbar!" - God is great - they shouted. The fighters then took down a Syrian flag flying in the centre of the square and set it on fire to the backdrop of incessant shooting.
The video appeared genuine and consistent with AP reporting on Idlib's takeover.
On its Twitter account, Nusra posted pictures of the Clock Tower and other landmark locations now under its control.
The Nusra Front is leading a group of ultra-conservative rebels in a major offensive that began earlier this week to take Idlib.
They include the hardline Ahrar al-Sham and Jund al-Aqsa groups and a few smaller groups loosely affiliated with the Free Syrian Army.
With the takeover of Idlib, an island of government territory in the midst of mostly opposition terrain, the Nusra Front further cements its hold over an impressive stretch of land it controls from the Turkish border to central and southern Syria.
With the world's attention focused on the Islamic State group, the Nusra Front has quietly consolidated its power in Syria in recent months, crushing moderate rebel groups the West may try to work with while increasingly enforcing its own brutal version of Islamic law.
Idlib, besides being a major population city, is located near the main highway linking the capital Damascus with Aleppo.
The main Western-backed Syrian National Coalition opposition group said the wresting of Idlib from government control is an "important victory on the road to the full liberation of Syrian soil from the Assad regime and its allies". However, it said more "decisive" assistance to Syrian rebels was needed for that to happen.
The Britain-based Syrian Observatory for Human Rights said rebel fighters seized control of Idlib after rapidly collapsing government forces withdrew.
The group, which relies on an extensive network of activists across Syria, said some fighting continued amid heavy artillery shelling from both sides. The Local Co-ordination Committees, another opposition activist collective in Syria, also reported the "almost complete" capture of Idlib by rebels.
An unnamed Syrian military official quoted by state-run news agency SANA said army forces were fighting "fierce battles" against "armed terrorist groups" to regain control in Idlib.
The government claimed earlier this week that "thousands of terrorists" streamed in from Turkey to attack Idlib and its suburbs. Turkey is one of the main backers of the rebels.
The humiliating losses in Idlib mark the second blow to government forces this week, after rebels, also led by Nusra, captured the ancient and strategic town of Busra Sham in southern Syria.
Also today, UN secretary-general Ban Ki-moon said he was angry and shamed by the failure of the world to stop Syria's raging civil war. He promised to step up diplomatic efforts in comments at a summit of Arab leaders in the Red Sea resort city of Sharm el-Sheikh in Egypt.
More than 220,000 people have been killed in the conflict, which began with popular protests amid Arab Spring uprisings in March 2011 and turned into an insurgency following a brutal military crackdown.