Huge blazes erupted in Syria's Aleppo as the city was rocked by fighting and air strikes on Thursday, ahead of last-ditch efforts by world powers to salvage a failed ceasefire.
The top diplomats from the United States and Russia were to meet with other key players in New York later Thursday, after UN Secretary General Ban Ki-moon said Syria's peace process was facing a "make or break moment".
The truce deal brokered by Moscow and Washington fell apart earlier this week, ushering in a surge of fighting on all major fronts of Syria's five-year civil war.
Heavy clashes gripped the outskirts of Aleppo on Thursday, after air strikes triggered major fires across the city's devastated rebel-held districts.
An AFP correspondent in the eastern Bustan al-Qasr neighbourhood reported that his entire street was in flames following the pre-dawn strikes.
Volunteer firefighters battled throughout the night to contain the blazes, which local activists at the Aleppo Media Centre said were caused by "incendiary phosphorous bombs."
In footage posted by the group, a ball of flames shoots up over the city, lighting up the skyline and sparking fires on the horizon.
The Syrian Observatory for Human Rights said 14 strikes on the rebel-held neighbourhoods of Bustan al-Qasr and Al-Kalasseh "led to massive fires" overnight.
Observatory head Rami Abdel Rahman said they were "the most intense strikes in months" on those two districts and that they had killed seven people, including three women and three children.
Widespread clashes 
The Britain-based monitor also reported fierce clashes in Aleppo's southwestern district of Ramussa, where rebel groups are fighting off a government offensive.
Fighting was also reported on Thursday in the central provinces of Homs and Hama, and east of Damascus in the opposition stronghold of Eastern Ghouta.
As violence escalated on the ground, diplomatic efforts were set to continue in New York with a new meeting of the International Syria Support Group (ISSG) on Thursday.
The ISSG, chaired by Moscow and Washington, met for an hour earlier this week but made little headway in agreeing on the next steps to end the war that has killed 300,000 people.
Moscow is a key ally of President Bashar al-Assad while Washington has supported moderate rebel groups opposed to his regime.
The latest Syria truce deal was reached after marathon talks between US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov earlier this month in Geneva.
It called for an end to fighting between regime forces and non-jihadist rebels, excluding extremists like the Islamic State group, as well as increased aid deliveries.
The truce broke down on Monday night, when Syria's military announced an end to a week-long freeze on fighting.
Kerry calls for grounded planes 
In his address to the Security Council on Wednesday, Lavrov said there would be "no more unilateral pauses" by Syrian government forces, arguing that rebels had previously used ceasefires to regroup.
Sounding a cautious note, Kerry said late Wednesday: "It's going to be difficult. We'll see what people are willing to do."
During his own impassioned address to the Security Council, Kerry demanded that Russia force its ally in Damascus to ground its air force in the wake of a deadly raid on an aid convoy in northern Syria.
Kerry said the strike -- which killed about 20 civilians and destroyed 18 aid trucks -- raised "profound doubt" about whether Russia and its Syrian ally were committed to upholding a ceasefire.
The strike was furiously condemned by aid organisations and the United Nations briefly halted its aid operations, before announcing it was ready to resume them on Wednesday.
Moscow denies that Russian or Syrian planes carried out the raid and instead said a coalition drone was in the area when the aid trucks were hit.
Russia's foreign ministry said the "unsubstantiated, hasty accusations" seemed designed to "distract attention from the strange 'error' of coalition pilots," referring to a US-led strike that killed dozens of Syrian soldiers at the weekend.