A regime warplane was shot down by opposition forces in western Syrian province of Hama on Saturday night, Britain based Syrian Observatory for Human Rights (SOHR) said.
The incident came as opposition and regime representatives set to attend UN brokered peace talks in Geneva set to resume on Monday.
There were conflicting accounts about the way and by which group the jet was brought down.
The SOHR said Ahrar al Sham, one of the main opposition groups fighting Bashar al Assad regime, fired two heat-seeking missiles at the military MiG-21 jet flying over Kafr Nabuda village, while another group operating in the area, Jaish al Nasr, said it had brought down the plane with anti-aircraft guns.
A cessation of hostilities deal brokered by US and Russia has been holding in Syria since February 27 despite violations reported on both sides.
Both Syrian regime and and main opposition organisation High Negotiations Committee (HNC) said they will attend the talks but differences between the both sides are dimming the chances of peace.
Regime’s Foreign Minister Walid Muallem ruled out discussion of Assad's future in the talks.
"We will not talk to anyone who talks about the position of the presidency. Bashar is a red line, the property of the Syrian people," Moualem said.
"I advise them that if this is their thinking, they shouldn't come to the talks."
He also criticised UN envoy Staffan de Mistura for already presenting an agenda for the talks and for saying that a presidential election would take place in 18 months.
"The government delegation will reject any attempt to put this on the agenda," Moualem told a televised news conference.
Within hours, opposition negotiator Mohammed Alloush, already in Geneva, had described Muallem's comments as worthless.
"We consider that the transitional period starts with the fall of Bashar al Assad or his death," he told reporters. "There's no possibility to start this period with the presence of this regime or the head of this regime in the power."
A UN Security Council resolution approved in December called for the establishment of "credible, inclusive, and non-sectarian governance," a new constitution and free and fair elections within 18 months.
The resumption of the Geneva talks will also mark the fifth-year anniversary of the civil war that has killed 470,000 people.
The war has also displaced more than half of Syrian population, around 5 million of which became refugees in other countries.