Syria's opposition says it agrees to the "possibility" of a temporary truce, provided there are guarantees Damascus's allies, including Russia, will cease fire.

Key points: Sieges are lifted, aid deliveries allowed

Sieges are lifted, aid deliveries allowed No signs of Moscow stopping its fight against "terrorists"

No signs of Moscow stopping its fight against "terrorists" UN special envoy says discussions are "intensive and constructive"

UN special envoy says discussions are "intensive and constructive" Opposition agrees to truce if Nusra Front no longer attacked

Sieges have been lifted and aid deliveries allowed country-wide, but there has been no immediate sign Moscow would drop its vow to continue its onslaught against those it views as "terrorists" among the range of armed groups pitted against President Bashar al-Assad.

President Assad himself added his own conditions to any truce plan, saying he was ready for a ceasefire on condition that "terrorists" did not use a lull in fighting to their advantage, and that countries backing insurgents halted support for them.

He especially singled out Syria's neighbour Turkey.

Throughout the five-year-old war, Damascus has routinely referred to all insurgents fighting the Government and its allies as terrorists.

To the frustration of the US and its allies, who support rebels trying to topple the President, Russian air strikes have turned the tide of the civil war in Mr Assad's favour.

Attempts to negotiate a truce in recent months have failed. The latest round of talks at the United Nations in Geneva is being jointly chaired by Russia and the US.

UN special envoy Staffan de Mistura said the discussions were "intensive and constructive", Swedish newspaper Svenska Dagbladet reported.

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.

But the Committee, which brings together a number of 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.

Al Qaeda-linked Nusra Front in spotlight

A source close to peace talks earlier said 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 the Islamic State, the source said.

It would be conditional on the Al Qaeda-linked Nusra Front 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 source close to peace talks said Russia had so far rejected the idea of halting attacks on Nusra.

Russian President Vladimir Putin's spokesman Dmitry Peskov said Russia was "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 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.

US Secretary of State John Kerry told his Russian counterpart Sergei Lavrov by phone that Washington was deeply concerned about Russia's continued bombing of civilian targets, a State Department spokesman said.

No US-Russia agreement

While the men restated their commitment to continued deliveries of aid, there was as yet no agreement on how a ceasefire would work, Mr Kerry said.

In Moscow, 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 states and armed groups embroiled in the war add to pessimism about the chances of success of further peace talks envisaged soon in Geneva.

Aid has finally been delivered to besieged Syrian towns in recent weeks. ( AFP: Louai Beshara; file photo )

Turkish Prime Minister Ahmet Davutoglu 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.

An official online media outlet for the Syrian presidency quoted Mr Assad as saying: "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."

He said any truce must ensure that "other countries, especially Turkey, are prevented from sending more terrorists and weapons, or any kind of logistical support".

Fighting raged on Saturday in Syria's Aleppo province, with Government forces claiming the capture of 18 villages from the Islamic State.

Reuters