Syrian government forces and their allies have fought insurgents near Aleppo and jets conducted raids around a nearby town seized by Islamist rebels, as Syria's military said it would extend a ceasefire in Aleppo by 48 hours.
A recent surge in bloodshed in Aleppo, Syria's largest city before the war, wrecked the 10-week-old, partial truce sponsored by Washington and Moscow that had allowed UN-brokered peace talks to convene in Geneva.
The US and Russia, which support rival sides in the civil war, said they would work to revive the February "cessation of hostilities" agreement that reduced fighting in parts of the country for several weeks.
US Secretary of State John Kerry said all parties had to press the sides they back to turn "words on a piece of paper" into actions to reinstate the truce.
Syria's military high command was quoted by state news agency SANA as saying the Aleppo ceasefire would be extended by 48 hours in the northern city beginning at 1am local time on Tuesday.
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Asaad al-Zoubi, the chief negotiator for the main Syrian opposition at the Geneva talks, criticised the extended Aleppo truce, telling Al Jazeera television that such measures served only to allow thousands of reinforcing troops to be sent from Iran, which is supporting Syrian President Bashar al-Assad.
Warplanes struck the town of Khan Touman, southwest of Aleppo, the British-based Syrian Observatory for Human Rights said. Rebels also fought government forces east of Damascus, and jets hit the rebel-held towns of Maarat al-Numan and Idlib.
Russia and the US said in a joint statement they would step up efforts to persuade the warring parties to abide by the ceasefire agreement.
"We have decided to reconfirm our commitment to the (ceasefire) in Syria and to intensify efforts to ensure its nation-wide implementation," they said.
"We demand that parties cease any indiscriminate attacks on civilians, including civilian infrastructure and medical facilities."
Visiting Paris, Kerry said a reduction of violence in line with the US-Russian joint statement depended on field commanders as well as interested parties including the US.
"These are words on a piece of paper. They are not actions," he said. "We have a responsibility to make certain that the opposition lives up to this, and Russia and Iran have a responsibility to make sure the Assad regime lives up to this."
Basma Kodmani, a member of the opposition High Negotiations Committee, expressed hope of a return to the Geneva peace talks if the US-Russian agreement is swiftly implemented.
The Observatory said warplanes struck rebel-held areas of the city early on Monday, and rebels fired shells into government-held neighbourhoods.
Al Manar, the television channel of Damascus's Lebanese ally Hezbollah, said on Monday troops had destroyed a tank belonging to insurgents and killed some of its occupants.
On the eastern edge of Damascus, government forces and their allies shelled rebel areas and clashed with insurgents, the Observatory and the rebel force Jaish al-Islam said. Three people were killed and 13 wounded in air strikes on Idlib, it said.
Jaish al-Islam agreed with a rival rebel group, Failaq al Rahman, that both would vacate a town they have been fighting over for almost two weeks, the Observatory said.
French Foreign Minister Jean-Marc Ayrault, hosting a meeting in Paris of Assad's opponents, said Syrian government forces and their allies had bombarded hospitals and refugee camps.
"It is not Daesh (Islamic State) that is being attacked in Aleppo, it is the moderate opposition," he said.
The US-Russian joint statement said Moscow would work with Syrian authorities "to minimise aviation operations over areas that are predominantly inhabited by civilians or parties to the cessation".
