Syria's regime said Monday it had been invited to peace talks in Geneva from March 14 but the opposition said it was still considering whether to attend despite a major lull in fighting.
The United Nations is hoping to restart peace talks that collapsed last month, building on a ceasefire that has led to the first significant decline in violence in Syria's nearly five-year civil war.
A source close to the Syrian regime delegation told AFP it had been invited to a new round of talks starting from March 14 in the Swiss city.
The UN's Syria envoy Staffan de Mistura has said he hopes the talks can begin from Thursday but officials have indicated it could take several days of preparations for the negotiations to get off the ground.
President Bashar al-Assad's regime has already expressed its willingness to take part in talks but the opposition has sent mixed signals on whether it will attend this latest round.
The head of the Riyadh-based High Negotiations Committee, Riad Hijab, on Monday refused to commit to the talks.
"The HNC will assess the situation in the coming days and we will take the appropriate decision," he told reporters.
Speaking in a conference call from Riyadh, Hijab said a small delegation from the HNC would travel to Geneva "in the next two days" to meet with the international task force monitoring the truce.
Hijab's statements appeared to be a step back from earlier comments by HNC spokesman Riad Naasan Agha, who said the opposition delegation would arrive on Friday to take part in talks.
The truce between President Bashar al-Assad's regime and non-jihadist rebels is part of the biggest diplomatic effort yet to resolve Syria's conflict, which has killed more than 270,000 people and forced millions from their homes.
Russia and the United States are on opposing sides of the conflict -- Moscow backs Assad and Washington supports the opposition -- but the two powers have made a concerted push for the truce and further peace efforts to succeed.
Observers say the partial truce, which does not apply to the Islamic State group or the Al-Qaeda-affiliated Al-Nusra Front, has largely held despite widespread scepticism before it took effect.
The Syrian Observatory for Human Rights, a Britain-based monitor, said Sunday had been the "calmest day" in Syria since the ceasefire began.
Observatory chief Rami Abdel Rahman told AFP that the average number of civilian deaths a day had fallen by 90 percent since the truce came into force, with an 80 percent decline among soldiers and rebel forces.
Aid deliveries have also improved, with the UN delivering aid on Monday to a key rebel bastion east of Damascus, the third distribution since truce began on February 27.
World Food Programme spokesman Hussam al-Saleh told AFP that 22 trucks would distribute food, flour and medical supplies to the towns of Hammuriya, Jisreen, and Beit Sawa in Eastern Ghouta.
Residents have taken advantage of the continued drop in fighting to stage daily protests.
In Aleppo city, dozens of young men poured into the street carrying the three-starred, tricolour flag of Syria's uprising and banners reading: "The world's silence is louder than the barrel bombs of death."
Moscow, which has provided a daily account of ceasefire violations, said Monday the truce was still "in general" holding apart from unspecified "isolated provocations and shelling".
It said Russian planes were continuing to carry out air strikes against IS and Al-Nusra in three provinces, including on the main IS stronghold of Raqa.
Russia launched an air campaign in September it says is targeting "terrorists" but has been accused of hitting non-jihadist rebels in support of Assad's forces.