Regime and rebel bombardment killed 26 civilians on Sunday in Syria's second city Aleppo as US President Barack Obama urged the conflict's warring parties to "reinstate" a troubled ceasefire.
Eight weeks into the declared truce between President Bashar al-Assad's regime and non-jihadist rebels, violence has escalated around Aleppo, with dozens killed by government air strikes and rebel rockets.
The surge in fighting and stalled peace talks in Geneva have dimmed hopes that the ceasefire would lay the groundwork for finally resolving Syria's devastating five-year conflict.
On Sunday, Obama said he had spoken to his Russian counterpart - a key Assad ally - to try to shore up the truce.
"I spoke to President Vladimir Putin early last week to try to make sure that we could reinstate the cessation of hostilities," he told reporters in Germany.
An EU spokesperson, in a statement, also urged the US and Russia as brokers of the ceasefire "to bring the maximum influence to bear in order to end these breaches of the agreement".
After at least 27 reported civilian deaths in regime bombardment across Syria on Saturday, a fresh barrage of air strikes hit Aleppo around midday Sunday.
Twelve civilians died after a strike hit an open-air fruit and vegetable market, the Syrian Observatory for Human Rights monitoring group said.
An AFP photographer saw a man in a bright-blue cap carrying a shell-shocked, bleeding and barefoot young boy.
Emergency responders, known as White Helmets, told AFP they were "exhausted" by the past three days of bombing in Aleppo city.
"We're back to working 24-hour shifts after we started working shorter hours because of the truce," one volunteer said.
According to the Observatory, four more civilians died Sunday in strikes on other opposition neighbourhoods.
In Aleppo's western government-held parts, 10 civilians including a woman and two children were killed early Sunday by rebel rocket fire, the Observatory said.
A regime air strike on the Salhin district also damaged the main water main, cutting off the city's supplies, local authorities said.
"The Assad regime's bombing offensive is not only a brutal attack on Syrians, but an attack on the Geneva process that is the only possible pathway to peace," HNC spokesman Salem al-Meslet said on Sunday in an emailed statement.
The truce was part of the biggest diplomatic push yet to resolve Syria's conflict, which began in March 2011 with widespread anti-Assad protests.
In an interview with the BBC aired Sunday, Obama warned Western governments should not send troops to topple Assad's regime.
"Syria has been a heartbreaking situation of enormous complexity, and I don't think there are any simple solutions," he said in London. "It would be a mistake for the United States, or Great Britain, or a combination of Western states to send in ground troops and overthrow the Assad regime."
He urged all parties "to sit down at the table and try to broker a transition".
Obama first urged Assad to step down in August 2011 but has resisted calls by critics to use American military force to end the Syrian regime's rule.