World powers have been pushing for a so-called cessation of hostilities to pave the way for renewed negotiations aimed at ending Syria's war nearly five years after it began.
But the truce has proved elusive so far, while fighting has intensified on the ground in northern Syria as tensions simmer between Russia and opposition backer Turkey.
UN-led talks on the ceasefire scheduled for Saturday have been postponed indefinitely, raising fresh concerns the day after US and Russian officials failed to agree concrete details on how it would be implemented.
Efforts to find a political solution to Syria's civil war collapsed earlier this month, and UN envoy Staffan de Mistura has said a plan to restart them on February 25 was no longer "realistically" possible.
Multiple groups are jostling for territory in northern Syria, with Russia pledging to keep up its air campaign supporting President Bashar al-Assad as the regime pressed its offensive around the Aleppo region.
Moscow on Saturday said it would continue "to provide assistance and help to the armed forces of Syria in their offensive actions against terrorists".
Russia also said it was "concerned at the growing tension at the Syrian-Turkish border," as Turkey continued shelling Kurdish-led forces in northern Syria it blames for a bomb attack in Ankara that killed dozens.
Assad also hit out at Turkey, telling Spanish daily El Pais he was "ready" for a ceasefire but only if "the terrorists" did not exploit it to improve their positions.
"It's about preventing other countries, especially Turkey, from sending more recruits, more terrorists, more armaments, or any kind of logistical support to those terrorists," according to a transcript on the daily's website.
- Opposition sets truce terms -
Speaking for Syria's opposition, the head of the High Negotiations Committee (HNC) umbrella group said they would agree to a temporary truce only if regime backers halted fire.
Riad Hijab said any ceasefire must be reached "with international mediation and with guarantees obliging Russia, Iran and their sectarian militias and mercenaries to stop fighting".
"There will not be a truce unless fighting stops simultaneously on the part of all the belligerents, sieges are lifted, humanitarian aid is delivered to those in need, and prisoners, particularly women and children, are released," Hijab said.
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More than 260,000 people have been killed in Syria since the conflict began, with half the population displaced, including over four million overseas.
Fighting has become increasingly complex, with Turkey firing artillery rounds into northern Syria for the past week in a bid to stem the advances of a Kurdish-led coalition that has seized territory from rebels.
The UN Security Council on Friday rejected a Russian bid to halt Turkish military action in Syria and plans for foreign forces to intervene on the ground, a decision met with "regret" from Moscow.
Ankara fiercely opposes both the regime and the Kurds, who it fears want to unite several Kurdish-majority regions in north and northeast Syria to create their own territory on its border.
The People's Protection Units (YPG) militia has led the joint Kurdish-Arab force that has swept through parts of Aleppo province in recent days, taking territory from Turkey-backed rebels.
Turkey has blamed the YPG and the Syrian Kurdish Democratic Union Party (PYD) for an attack in the capital that killed 28 people days ago, although they both deny it and another Kurdish group has claimed the bombing.
- Regime gains -
Meanwhile, Syrian regime forces have also advanced in Aleppo province, backed by Russian air strikes.
On Saturday, government troops seized 18 villages around the road east from Aleppo city towards the Islamic State group stronghold of Raqa, the Syrian Observatory for Human Rights monitor said.
The advances secure the regime's grip on some 40 kilometres (25 miles) of the highway, which passes by the Kweyris military airport that government forces recaptured with Russian help in November.
The Syrian opposition and its backers accuse Moscow of focusing on moderate and Islamist rebels rather than jihadists such as IS.
US Secretary of State John Kerry on Saturday said a ceasefire needed to be agreed as quickly as possible in a phone conversation with Russian Foreign Minister Sergei Lavrov.
"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," State Department spokesman John Kirby said.