An Islamic State group suicide bomber killed 51 people outside Syria's Al-Bab on Friday, in a major blow just hours after rebels seized the town from the jihadists.
In a statement distributed online, the group said its suicide bomber "drove his car bomb into the middle of a gathering of Turkish soldiers and apostates working in Susian".
At least 51 people were killed in the blast, most of them civilians, according to the Syrian Observatory for Human Rights.
The monitoring group said the car bomb had targeted twin command posts at a rebel base in Susian, about eight kilometres (five miles) from Al-Bab.
Rebel field commander Abu Jaafar told AFP that fighters, Turkish soldiers, and civilians had gathered at Susian for an overnight meeting "to organise a security apparatus and set a plan for rebuilding Al-Bab".
"This information reached the (IS) sleeper cells, which prepared a car bomb" that detonated at Susian at around 0800 am (0600 GMT), he told AFP.
Abu Jaafar, who was near Susian at the time of the attack, said hospitals in the area were full of wounded.
"(IS leader Abu Bakr) al-Baghdadi's dogs could not bear their huge loss, and their suicide bombers have begun to take revenge," he said.
Separately, two Turkish soldiers were killed in a suicide attack in Al-Bab on Friday as they were carrying out road checks, Prime Minister Binali Yildirim said.
- Stray cats, battered streets -
The strategic town, just 25 kilometres (15 miles) south of the Turkish border, was the jihadists' last stronghold in the northern Syrian province of Aleppo.
An AFP correspondent inside Al-Bab said stray cats were on the bombed-out streets Friday, sniffing at the bodies of alleged IS fighters.
Doors of shops in the town's main marketplace had been blown open by bomb blasts, and boxes of food and medicine were scattered on the ground.
A rebel scrawled the name of his battalion -- "Mutasem Brigades" -- on a wall.
In the town's south, rescuers and bulldozers were pulling bodies from under the rubble, careful not to detonate any mines left behind by IS.
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Turkey sent troops into Syria last August in an operation it said targeted not only IS but also US-backed Kurdish fighters whom it regards as terrorists.
With its support, the rebels launched an offensive to take Al-Bab last year.
It has proved the bloodiest battle of Ankara's campaign accounting for most of the 71 Turkish losses so far.
On Friday the Turkish army said in a statement that "control of all neighbourhoods in Al-Bab have been secured" by opposition fighters supported by Ankara.
The town was also seen as a prize by Syrian President Bashar al-Assad's forces, who had advanced to just 1.5 kilometres (one mile) from its outskirts in recent weeks.
The battle against IS around Al-Bab is just one front line in Aleppo province.
- Rebels, regime pound Aleppo -
West of the second city, where government forces took full control in December, fighting flared with rebels in its western suburbs even as peace talks got under way in Geneva.
Exchanges of rocket and artillery fire first broke out on Wednesday, centred on the rebel-held district of Rashideen, the Observatory said.
The government responded with intensive air strikes on Thursday that killed at least 32 rebels.
A fragile ceasefire between government forces and non-jihadist rebels has been in force since late December, brokered by regime ally Russia and rebel backer Turkey.
It has led to a sharp reduction in fighting in many areas.
But parts of the country which are held by IS or its jihadist rival, former Al-Qaeda affiliate Fateh al-Sham Front, are not covered by the truce.
The talks in Geneva between government and opposition representatives formally opened on Thursday.
They are the fourth round of UN-brokered negotiations, aimed at ending a conflict that has dragged on for nearly six years and claimed more than 310,000 lives.
UN envoy Staffan de Mistura said: "I'm not expecting miracles," but warned of dire consequences if the talks "fail again".