Rebels shot down a Syrian warplane in the west of the country, fighters and a military source said, as the government and opposition groups traded barbs ahead of fragile peace talks in Geneva on Monday.
There were conflicting accounts on Saturday night whether the government plane was brought down by missiles or anti-aircraft guns.
The Syrian military source told the Reuters news agency that fighters targeted a warplane while it was landing in Hama province, bringing it down, and calling the attack a breach of the "cessation of hostilities" agreement.
A rebel group operating in the area, Jaish al-Nasr, said it shot down the jet with anti-aircraft weapons. The Syrian Observatory for Human Rights, however, said rebels downed the plane with two heat-seeking missiles.
The incident came as the Syrian government confirmed it will attend indirect talks with the opposition in Geneva on Monday.
The negotiations have been brokered by the UN's Syria envoy Staffan de Mistura, who said discussions will include the implementation of presidential elections within 18 months.
In an interview with Al Jazeera's diplomatic editor James Bays, de Mistura said the political process will include a push for "new governance", including a new constitution and a new election under UN supervision.
Syrian Foreign Minister Walid al-Muallem said on Saturday, however, the government delegation in Geneva will reject any attempt to include presidential elections on the agenda.
"Neither he [de Mistura] nor anyone else, whoever they may be, has the right to discuss presidential elections. This right is exclusively for the Syrian people," Muallem told a news conference in Damascus.
"We will not talk with anyone who wants to discuss the presidency... [President] Bashar al-Assad is a red line and is the property of the Syrian people."
He added the delegation to the talks will wait no more than "24 hours" for the opposition to arrive.
The main opposition bloc, the Riyadh-based High Negotiations Committee (HNC), has repeatedly called for Assad's departure at the start of any transitional period.
"We consider that the transitional period begins with the departure of Bashar al-Assad or his death. It cannot be a stage where this regime, or the head of this regime, continues to be in power," said Mohammed Alloush, HNC's senior negotiator.
The talks will coincide with next week's fifth anniversary of a war that has killed more than 250,000 people, created the world's worst refugee crisis, and allowed the expansion of the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) armed group.
They are part of the first diplomatic push since the Russian air force intervened in September to support Assad, tilting the war his favour and helping Damascus reclaim significant areas in the west.
The ceasefire agreement, brokered by the United States and Russia, has been more widely respected than many expected, though fighting has continued on some important fronts, including near the Turkish border.
Source: Al Jazeera and agencies