A number of Syrian opposition groups have declared that they agree to the "possibility" of a temporary truce if President Bashar al-Assad's government and its allies respect several conditions, including halting fire.
The groups said on Saturday they would agree provided there were guarantees that the Syrian government forces and its allies would respect a ceasefire, sieges were lifted and aid deliveries permitted across the country.
The announcement came as fighting continued on the ground despite a Friday deadline for cessation of hostilities.
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Russian fighter jets are repeatedly striking rebel targets, particularly in Aleppo, backing government forces as they push towards Syria's second city.
The opposition factions "expressed agreement on the possibility of reaching a temporary truce deal, to be reached through international mediation", a statement from the High Negotiations Committee said.
It said the UN must guarantee "holding Russia and Iran and sectarian militias ... to a halt to fighting".
All sides should cease fire simultaneously and the government should release prisoners, the statement said.
The UN is struggling to deliver aid to about 4.5 million Syrians who live in hard-to-reach areas, including nearly 400,000 people in besieged areas.
The fighting in Syria started as an unarmed uprising against Assad in March 2011, but has since expanded into a full-on conflict that has killed more than 260,000 people, according to UN estimates.
Russia said on Saturday it would continue "to provide assistance and help to the armed forces of Syria in their offensive actions against terrorists".
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It also expressed "regret" that a planned UN Security Council resolution cautioning against a Turkish ground invasion was rejected on Friday, and said it was "concerned at the growing tension at the Syrian-Turkish border".
It said UN-led talks planned for Saturday between major international players on a ceasefire in Syria had been postponed.
Maria Zakharova, Russian foreign ministry spokeswoman, told Russian news agencies the meeting in Geneva had been put back to an unspecified later date as "consultations" between key nations continued.
Military and diplomatic officials from Russia and the US held talks on Friday to try to finalise the details of a possible ceasefire, as a hoped-for halt in hostilities on the ground failed to materialise.
That gathering was supposed to pave the way for a broader meeting after the 17 key international players involved in negotiations to end the Syrian conflict agreed on January 12 that a ceasefire should come into force within a week.
The truce failed to take effect on Friday as fighting continued in Syria, with Kurdish-led forces backed by US-led air power seizing a key town from the Islamic State of the Iraq and the Levant (ISIL) group.
John Kerry, US secretary of state, urged on Saturday that the ceasefire be agreed as soon as possible, in a phone conversation with Sergey Lavrov, Russia's foreign minister.
"Secretary Kerry also restated his deep concern over the indiscriminate nature of continued bombing by Russian military aircraft and the lives being lost as a result," John Kirby, state department spokesman, said.
Russia is currently flying a bombing campaign in Syria to back up forces loyal to Assad, while the US is leading a coalition against ISIL, which has seized territory in Syria and Iraq.
Syrian forces have also advanced in Aleppo province, backed by Russian air strikes.
Government troops on Saturday seized 18 villages around the road east from Aleppo city towards the ISIL stronghold of Raqqa, the monitoring group Syrian Observatory for Human Rights said.
The advances secure government forces' grip on about 40km of the highway, which passes by the Kweyris military airport that government forces recaptured with Russian help in November.
Source: Al Jazeera and agencies