At least 15 airstrikes were carried out on Saturday by Syrian regime aircraft in rebel-held areas in the northern city of Aleppo where more than 240 civilians have been killed in the past eight days, activists said.
The strikes targeted the opposition-controlled districts of Bustan al- Qasr, al-Ansari and Kalsa, they said.
The Syrian Observatory for Human Rights reported that at least two civilians were killed and an unspecified number injured.
Violence has simmered since April 22 in Aleppo, where at least 244 civilians have been killed in reciprocal attacks by regime forces and rebels, the Observatory said.
The dead included 43 children, according to the Britain-based watchdog that relies on a network of activists inside Syria.
"The regime planes focus their raids on areas where there are civilians," Mahmoud al-Shami, an activist inside Aleppo, told dpa via a Facebook message.
"They want to kill as many civilians as they can to raise fear and force the survivors to flee the area," he said.
An estimated 250,000 people remain in Aleppo, according to aid group Doctors Without Borders.
Aleppo, Syria's pre-war commercial hub, is divided between a government-held western sector and the rebel-held east.
Recent regime military advances, backed by Russian airstrikes, have put the east at risk of siege.
The Syrian government has excluded Aleppo from a temporary ceasefire that started early Saturday in the capital Damascus, the nearby area of Eastern Ghouta and the coastal area of Latakia.
On Friday, the Syrian army announced a cessation of hostilities in Damascus and Eastern Ghouta for 24 hours and in the Latakia countryside for 72 hours.
These areas were calm on Saturday, the Observatory reported.
The US State Department said that John Kerry would travel to Geneva to meet on Sunday and Monday with the foreign ministers of Jordan and Saudi Arabia, as well as UN Special Envoy for Syria Staffan de Mistura.
The US Secretary of State "will review ongoing efforts to reaffirm the cessation of hostilities nationwide in Syria," a statement said.
The overall level of violence in Syria was significantly reduced in the days following the declaration of a partial nationwide truce in February.
But the latest round of Syrian peace talks, which ended in Geneva last week having made little progress, were marred by the drastic escalation of fighting in Aleppo and elsewhere. Fresh talks on ending Syria's five-year war are due to resume in May.