BEIRUT (Reuters) - Syria's opposition on Saturday said it had agreed to the "possibility" of a temporary truce, provided there were guarantees Damascus's allies including Russia would cease fire, sieges were lifted and aid deliveries were allowed country-wide.
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 President Bashar al-Assad.
And Assad himself added his own conditions to any truce plan, saying on Saturday 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 neighboring 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 United States and its allies who support rebels trying to topple the president, Russian air strikes have turned the tide of the civil war in Assad's favor.
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 United States.
U.N. special envoy Staffan de Mistura said on Saturday 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 United Nations 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 earlier told Reuters 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 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 organization by the U.N. 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 organizations".
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.
U.S. 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 said.
While the men restated their commitment to continued deliveries of aid, there was as yet no agreement on how a ceasefire would work, John Kirby 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.
Turkish Prime Minister Ahmet Davutoglu on Saturday called on the United States 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 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 Islamic State.
Reporting by Tom Perry and John Davison in Beirut, Tom Miles in Geneva and Dmitry Solovyov in Moscow; Writing by Andrew Roche in London; Editing by Kevin Liffey