Syria’s opposition has agreed to a two- to three-week truce on condition Russia stops its airstrikes, sieges are ended and access for aid allowed across the country, a source close to peace talks said Saturday.

Russian airstrikes began last September and turned the tide in Syria's five-year civil war in Bashar al-Assad's favor, to the frustration of the United States and its allies who have supported rebels trying to topple the president.

Several attempts to agree a truce have failed in recent months.

“The message has been conveyed very clearly now that the opposition is willing to enter into a two- to three-week truce, and that’s open to renewal if the conditions are right and both sides are willing to renew it,” the source said.

The opposition also wants vulnerable prisoners, including women and children, to be released, the source said.

Russia and the U.S. are jointly chairing a United Nations meeting in Geneva aimed at trying to secure a cessation of hostilities in Syria.

U.S. and Russian military officials met ahead of the wider meeting, diplomats said on Friday.

A truce would be renewable and supported by all parties except Islamic State, the source said. It would also be conditional on the al Qaeda-linked Nusra Front no longer being targeted, at least to start with, the source said.

The Nusra Front is considered a terrorist organization by the U.N. Security Council and banned.

Asked if the opposition’s insistence on the Nusra Front no longer being targeted was the main stumbling block, he described it 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 in Idlib on their hands,” the source said.

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

Ending sieges on civilians have become another key sticking point in talks to end the conflict.

The U.N. estimates there are 486,700 people in around 15 besieged areas of Syria, and 4.6 million in hard-to-reach areas. In some, starvation deaths and severe malnutrition have been reported.