Barrel bombs and air strikes hit Syria's Aleppo on Thursday hours after a temporary truce announced by regime ally Russia came into effect in the northern war-torn city.
An AFP reporter said regime helicopters dropped the crude explosive devices on the city's rebel areas after residents had headed to markets for their first morning shopping in weeks.
In the evening air strikes hit the eastern part of the city controlled by the rebels, the Syrian Observatory for Human Rights said, adding that the insurgents retaliated by firing rockets into the regime-held west.
At least four civilians were killed in the fresh violence, three on the eastern side of Aleppo and one in the west, added the Britain-based monitor.
Aleppo has seen some of the worst fighting in a war that has killed more than 280,000 people, and there is deep scepticism that the latest halt to fighting in the battered city will last.
The two-day truce came hours after US Secretary of State John Kerry warned Moscow that Washington's patience was running out over breaches of a nationwide ceasefire.
Peace talks aimed at ending the five-year conflict have stalled and a February countrywide ceasefire between the regime and non-jihadist rebels lies in tatters.
"There is no progress in the political process," Russian Foreign Minister Sergei Lavrov said, referring to Syria.
He accused Washington -- which broadly supports the opposition -- of being "unable or unwilling to put pressure on its allies in the region".
Nevertheless, direct contact between Russia and the United States about Syria have taken place "without any hysteria", he added.
There have been repeated violations of the February 27 truce in Aleppo, with rebels pounding regime-controlled neighbourhoods with rocket and artillery fire and the regime hitting rebel areas with air strikes.
- 'Artificial and fruitless' -
The new ceasefire was announced by Moscow late Wednesday in a bid to halt violence in the city, split since 2012 between a regime-held west and a rebel-controlled east.
"On Russia's initiative, a 'regime of silence' has been introduced in Aleppo for 48 hours from 00:01 16 June (2101 GMT Wednesday) with the goal of lowering the level of armed violence and stabilising the situation," Russia's defence ministry said.
A source close to the regime said the truce had been decided "in connivance" with Washington.
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Residents in Aleppo's rebel-held east had headed out to the markets to buy meat and vegetables on Thursday morning, the AFP correspondent said.
After weeks of air strikes, it was the first time they had shopped in the morning since the start of the holy Muslim month of Ramadan.
A Syria analyst, Karim Bitar, described the latest Aleppo truce as a "ceasefire of convenience, which is not linked to a real political process".
"Syrians are increasingly sceptical about these brief ceasefires, which seem to have become as artificial and fruitless and the negotiation sessions -- which each time revive hopes then end in bitter disappointment."
- 'Rest for the killers' -
The Observatory, which relies on sources inside Syria to gather information on the war, agreed.
"This temporary truce of a few hours doesn't aim to end the bloodshed, but to give some rest to the killers -- those who massacre the people of Aleppo and Syria -- before they resume their crimes," it said in a statement.
A US-led coalition has been bombing the Islamic State group -- which is not included in the February ceasefire -- since 2014.
CIA chief John Brennan said on Thursday that IS has suffered major losses as a result, but that it retains the ability to conduct attacks around the world.
UN-backed Syria peace talks in Geneva came to a deadlock in April after the opposition walked out over increasing violence and lack of aid access to besieged areas.
The United Nations says nearly 600,000 Syrians live in besieged areas, most surrounded by government forces.
An aid convoy was ready to go to one of these areas in the central province of Homs and was expected to ferry in desperately needed aid there Thursday, the UN said.
"We are ready loaded with a large convoy to go to the besieged town of Al-Waer," said Jan Egeland, who heads a UN-backed humanitarian taskforce for Syria.
If the convoy entered, it would be of "great significance", he said, adding the rebel-held area had been "without supplies for more than three months."
"People have died in Al-Waer because of lack of humanitarian supplies of late," he said.