A suicide bomber from the Al-Nusra Front terrorist group, Syria's Al-Qaeda offshoot, blew himself up in a Syrian army outpost in a contested neighborhood in the city of Aleppo on Monday night.
The bombing killed at least 25 soldiers and allied militia and injured scores, according to the Reuters news agency.
The Britain-based Observatory for Human Rights, which tracks the violence across the country, said the explosion from the blast was heard across the city where several Islamist-led groups recently launched an offensive to control the remaining western parts of the city in government hands.
Rebel websites said there was heavy bombardment of rebel-held portions of Jamiyat al Zahra district, where rebels had gained ground and brought them closer to the heart of government controlled parts of the city, which was before the 2011 civil war the country's main industrial and commercial hub.
Rami Abdul Rahman, head of the Observatory, was quoted by Reuters as having said that the Syrian army had responded with heavy firepower including aerial bombardment, in an attempt to stem the offensive.
The insurgents posted on social media images of fighters from the Al-Nusra Front heading towards the district, which the Syrian army has repeatedly said in recent days had not fallen into insurgent hands.
Al-Nusra Front has been an official branch of Al-Qaeda since April 2013, and dominates a swathe of northwest Syria despite months of international airstrikes.
As well as fighting President Bashar Al-Assad's regime, the group has clashed with Western-backed rebels and its jihadist rival the Islamic State (ISIS) group.
Al-Nusra Front recently reaffirmed its allegiance to Al-Qaeda and denied any plan to break away and become a more internationally acceptable rebel force, amid speculations on Internet social networks of a split between the jihadist allies.