Syria’s opposition has agreed to the possibility of a temporary truce in the civil war, if there are guarantees that Damascus’s allies including Russia cease fire, sieges are lifted and aid deliveries are allowed nationwide.

But there was no immediate sign that Moscow would drop its vow to continue its onslaught against those it views as “terrorists” among the range of armed groups pitted against the president, Bashar al-Assad.

Assad responded on Saturday by saying he was ready for a ceasefire on condition “terrorists” did not use a lull in fighting to their advantage and that countries backing insurgents halted support for them.

“We have said that we are ready to stop military operations, but the issue relates to more important factors … such as preventing terrorists from using it to improve their positions,” Assad told Spanish newspaper El Pais in an interview.

He also said any truce must ensure that “other countries, especially Turkey, are prevented from sending more terrorists and weapons, or any kind of logistical support”. Damascus refers to all insurgents fighting against the Syrian army and its allies as terrorists.

Russian airstrikes have turned the tide in Syria’s five-year-old civil war in Assad’s favour, to the frustration of the US and its allies who support rebels trying to topple the president. Attempts to negotiate a truce in recent months have failed. The latest round of talks at the UN in Geneva is being jointly chaired by Russia and the US.

On Saturday, various Syrian 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.

The committee, which brings together armed rebel groups and Syria’s exiled political opposition, 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 Syrian government should release prisoners, it added.

A source close to peace talks told Reuters earlier that Syria’s opposition had agreed to the idea of a two- to three-week truce. The truce would be renewable and supported by all parties except Islamic State, the source said.

It would be conditional on the Nusra Front, which is linked to al-Qaida, no longer being attacked by Syrian government forces and their allies, at least to start with, the source said. The Nusra Front is considered a terrorist organisation by the UN security council.

No halt to Russian bombardments looked immediately likely. The spokesman for Vladimir Putin, Dmitry Peskov, said on Saturday: “Russia is sticking to its consistent policy of rendering assistance and aid to the armed forces of Syria in their offensive actions against terrorists and against terrorist organisations.”

The source close to peace talks described the opposition’s insistence on the Nusra Front no longer being targeted as “the elephant in the room”.

“They have to deal with this very delicately or they are going to end up with a civil war [among rebel groups] in Idlib on their hands,” the source said.

Nusra fighters are fighting alongside other rebel groups in some areas, including Idlib.

The US secretary of state, John Kerry, told his Russian counterpart, Sergei Lavrov, by phone on Saturday that Washington was deeply concerned about Russia’s continued bombing of civilian targets. A State Department spokesman added that the men had restated their commitment to continued deliveries of aid but there was as yet no agreement on how a ceasefire would work.

The Russian foreign ministry said the two men had discussed how to implement a ceasefire that would exclude “operations to fight terrorist groups”.

The tangled alliances and rivalries among the factions in the war add to pessimism about the chances of success of further peace talks envisaged soon in Geneva.

The Turkish prime minister, Ahmet Davutoğlu, on Saturday called on the US to give unconditional support in the fight against Syrian Kurdish militants, illustrating growing tension between Ankara and Washington over policy in northern Syria.

Fighting continued to rage on Saturday in Syria’s Aleppo province, with Syrian government forces claiming the capture of 18 villages from Isis.

Assad said in the Spanish interview that his troops were close to fully controlling the northern city of Aleppo and were advancing towards the Islamic State stronghold of Raqqa province.

Once he had control of the country, Assad said, the next step would be to form a national unity government that would lay the groundwork for a new constitution and general elections.