Rebel-held areas of the northern Syrian city of Aleppo saw the heaviest air strikes in months activists say, as a week-old truce collapsed.
An AFP news agency correspondent reported that his entire road was left after incendiary bombs dropped on it overnight burning.
At least seven people, including three kids, are considered to have perished.
Fighting also erupted in the southern district, where rebels are attempting to break a siege by government forces.
Aleppo, once the commercial and industrial heart of Syria, has been split roughly in two with the government controlling rebels the east and the west.
Two million people are caught up in the battle for the city, and getting help to them had been a vital part of the cessation of hostilities deal brokered by the USA and Russia. However, no deliveries have taken place thus far.
A fatal attack on an aid convoy and Syrian Arab Red Crescent warehouse outside Aleppo on Monday, for which Moscow and Washington have blamed each other, prompted the UN to briefly suspend deliveries.
But on Thursday, a spokesman said it was sending a convoy into a besieged suburb of the capital, Damascus, and hoped to reach Aleppo “in the near future”.
The UK-based monitoring group’s director, Rami Abdul Rahman, described them as “the most intense strikes in months” on the two areas and said they’d left seven people dead, including three girls and three children.
An AFP journalist living in Bustan al Qasr said his whole road was in flames following the air strikes.
Volunteer firemen reportedly struggled to contain the fires, which the pro-opposition Aleppo Media Centre said were caused by “incendiary phosphorus bombs”.
Video footage posted by the activist group and the pro -opposition Thiqa news agency revealed extreme blazes lighting up the night sky.
The Syrian Observatory said the Amariya, Old City and Job 1070 areas also came under aerial attack, while rebel shells fell on government-controlled Bustan al Zahra and Mashariqa.
Fighting was also reported in the central states of Hama and Homs, and in the eastern Ghouta region outside Damascus, AFP said.
Russia and the US, which support Syria’s government and opposition respectively, are attempting to renovate the cessation of hostilities and will co chair a meeting of the International Syria Support Group (ISSG) in New York later on Thursday.