Syrian regime forces and their allies attacked opposition forces near Aleppo on Monday and jets conducted raids around a nearby town, a monitoring group said, as Syria's military said a ceasefire in Aleppo would be extended by 48 hours starting on Tuesday.
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 United States 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.
The UN said two weeks ago that the February 27 cessation of hostilities deal is "barely alive."
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 (2200 GMT on Monday).
A number of short-term local truces have been in place since April 29, first around Damascus and northern Latakia and then in Aleppo, but there has still been fighting between opposition and regime forces.
The cessation of hostilities and local truces do not include DAESH and Al Qaeda’s Syrian affiliate Nusra Front.
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 Assad regime.
Warplanes struck the town of Khan Touman, southwest of Aleppo, the British-based Syrian Observatory for Human Rights said. Opposition forces also fought regime forces east of Damascus, and jets hit the rebel-held towns of Maarat al-Numan and Idlib.
Russia and the United States 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 United States.
"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.
Aleppo has been the epicentre of a military escalation that has helped to undermine UN-led peace talks in recent weeks. A cessation of hostilities agreement has unravelled and fighting has resumed on numerous fronts in western Syria.
The city is divided into areas held by the government and opposition forces.
Around 400,000 people have been killed so far in the five-year wa in Syria, according to UN Special Envoy to Syria Staffan de Mistura.