The UN envoy for Syria called Friday for elections in the war-ravaged country in 18 months, as the opposition announced it will attend fresh peace talks next week.
But in a worrying development ahead of the negotiations, government raids were reported to have killed seven civilians in Syria's second city, Aleppo, despite a ceasefire.
The truce has prompted a nearly two-week lull in fighting between the Russian-backed regime and non-jihadist rebels since coming into force on February 27.
World powers are counting on the ceasefire to hold for a new round of indirect negotiations between the opposition and the government due to start on March 14 in Geneva.
The US State Department said Friday that the ceasefire is "largely holding," albeit with continued violations that include government attacks on civilians and opposition forces.
"Despite the reduction in violence nationwide, we remain deeply concerned by continued specific violations to the cessation of hostilities, including attacks on civilians and opposition forces by the regime and its supporters," spokesman John Kirby said.
The Riyadh-based High Negotiations Committee, the main Syrian opposition grouping, agreed on Friday to attend the UN-backed talks.
The HNC said its delegation would focus on creating a "transitional governance body with full executive powers".
It insisted President Bashar al-Assad "will have no place" in a future government.
A plan agreed by world powers last year called for six months of negotiations followed by a transitional government, a new constitution and elections within 18 months.
Last month Assad's regime announced it would hold parliamentary elections on April 13 instead, drawing criticism.
UN envoy Staffan de Mistura has said "substantive" talks will begin Monday in Geneva and last no longer than 10 days.
The first day of negotiations would start the countdown to both presidential and parliamentary elections in Syria under UN observation, he said.
"New elections... should be held 18 months from the start of talks, that is from March 14," de Mistura told Russia's RIA Novosti state news agency, in comments translated into Russian.
That would mean elections around mid-September 2017.
- 'Serious violation' -
In addition to planning the polls, the focus in Geneva will be on the formation of "an inclusive new government" and a new constitution, said the envoy.
"I hope that during the first stage of talks, we reach progress at least on the first question (of the new government), it doesn't matter whether this is on paper," he was quoted as saying.
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A source close to Syria's government told AFP this week that its delegation would be attending the talks.
Previous diplomatic efforts to resolve the complex conflict have failed.
The war, which is to enter its sixth year next week, has killed more than 270,000 people and displaced millions.
The chief prosecutor for the Yugoslav war crimes tribunal said Friday that those behind atrocities in Syria must eventually be held to account.
"As an international prosecutor and somebody who believes in justice... it is obvious that sooner or later accountability will be needed for the crimes committed in Syria," Serge Brammertz told AFP.
The last round of UN-sponsored talks collapsed in Geneva in February amid a fierce Russian-backed government offensive in Aleppo province.
Since then, regime fighters and rebels have largely abided by a partial truce that has seen a dramatic drop in air strikes, fighting and deaths.
- 'Glimmers of hope' -
The UN's top humanitarian chiefs welcomed "fragile glimmers of hope" in Syria after the ceasefire allowed more aid deliveries.
"Fewer bombs are falling; humanitarian access has opened up in some places; negotiators from all sides are preparing to come together and talk," they said in a joint statement.
But they warned this was "just not enough".
The Syrian Observatory for Human Rights described government air strikes that killed at least seven civilians in Aleppo city as "the most serious violation in the city since the truce came into effect."
An AFP correspondent in Aleppo said the raids struck a mosque.
Hundreds of demonstrators took to the streets across Syria after Friday prayers for the second week in a row.
In Maaret al-Numan, northwest Idlib province, dozens of protesters waving the three-starred, tricolour uprising flag briefly clashed with members of Al-Qaeda affiliate Al-Nusra Front.
In a video posted online, motorcyclists waving Al-Qaeda's black flag pulled up to the protest and attempted to drown out the singing with calls of "Allahu akbar" or "God is greatest."
But the crowd pushed the Al-Nusra members out, chanting, "The Syrian people are one!"
Al-Nusra leads an Islamist coalition that controls much of Idlib province and has seized activists and journalists in the past.