Rebel-held areas of the northern Syrian city of Aleppo saw the heaviest air strikes in months overnight, activists say, as a week-old truce collapsed.
An AFP news agency journalist reported that his entire street in the Bustan al-Qasr district was left burning after warplanes dropped incendiary bombs.
At least 13 people, including women and children, are believed to have died.
Fighting also erupted in the southern district, where rebels are attempting to break a siege by government forces.
Aleppo, once Syria's commercial and industrial hub, has been divided roughly in two since 2012, with the government controlling the west and rebels the east.
Two million people are caught up in the battle for the city, and getting aid to them had been a key part of the cessation of hostilities deal brokered by the United States and Russia. However, no deliveries have taken place so far.
A deadly attack on an aid convoy and Syrian Arab Red Crescent warehouse outside Aleppo on Monday, for which Washington and Moscow have blamed each other, prompted the UN to temporarily suspend deliveries across the country.
But following the pause a convoy on Thursday entered Muadhamiya, a suburb of the capital, Damascus, where some 40,000 people are living under siege, the UN tweeted.
A spokesman hoped the UN could reach Aleppo "in the near future".
The Syrian Observatory for Human Rights reported on Thursday that 14 air strikes had hit Aleppo's rebel-held southern Bustan al-Qasr district and neighbouring Kallasa, leading to "massive fires", amid clashes between rebel and troops.
The UK-based monitoring group's director, Rami Abdul Rahman, described them as "the most intense strikes in months" on the two areas, with three women and three children among the 13 dead.
The Aleppo Media Centre said the fires were caused by "incendiary phosphorus bombs". Video footage posted by it and another pro-opposition activist group, Thiqa, showed intense blazes lighting up the night sky.
The Syrian Observatory said the Amariya, Old City and Project 1070 areas also came under aerial attack, while rebel shells fell on government-controlled Bustan al-Zahra and Mashariqa.
Separately, the Observatory said 11 people had died and dozens were hurt in a suicide attack in the rebel-held village of Inkhil in southern Deraa province.
A key rebel figure and head of the Deraa provincial council, Yakoub al-Ammar, was among those killed, Syria's opposition National Coalition said.
Russia and the US, which support Syria's government and opposition respectively, are attempting to revive the cessation of hostilities and will co-chair a meeting of the International Syria Support Group (ISSG) in New York later on Thursday.
But the prospects for progress are thin, says the BBC's James Longman in Beirut.
In an address to the UN Security Council on Wednesday, Russian Foreign Minister Sergei Lavrov warned that there would be "no more unilateral pauses" by Syrian government forces.
US Secretary of State John Kerry meanwhile called for all military aircraft in Syria to be grounded in the wake of the aid convoy attack.
US officials told the BBC on Tuesday that their information clearly indicated that it was an air strike, and that two Russian Su-24 ground attack aircraft were in the sky above the convoy at the precise moment it came under fire.
Russian officials have vigorously denied the allegation and said that a US Predator drone was flying above the area at the time.
Syrian President Bashar al-Assad also defended his key ally on Thursday, insisting that whatever American officials said "had no credibility" and were "just lies".
In an interview with the Associated Press, he also said an air strike by the US-led coalition against so-called Islamic State in the eastern city of Deir al-Zour on Saturday, which killed dozens of Syrian soldiers, was "definitely intentional".
"It wasn't an accident by one airplane," he said.
"It was four airplanes that kept attacking the position of the Syrian troops for nearly one hour, or a little bit more than one hour. You don't commit a mistake for more than one hour."