Opposition negotiators have started arriving in Geneva in advance of Syria peace talks, which are expected to tackle the issue of President Bashar al-Assad's presidency.

The talks on Monday in the Swiss city 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.

Talk to Al Jazeera: 'No plan B for Syria', says UN envoy Staffan de Mistura

Al Jazeera's Diplomatic Editor James Bays, reporting from Geneva, said UN envoy Staffan de Mistura has openly stated he wants the talks to focus on "substantive issues", including a new constitution and UN-monitored elections.

De Mistura told Al Jazeera that under the current peace process,  there is a higher chance than ever of achieving a political solution in Syria.

"I think what is important this time around [compared to previous peace talks], is that we seem to be getting to the key crunch issue - the future role of President Assad and those close to him," said Bays.

Despite optimism about discussions to find a political solution to the deadly conflict, the Syrian government and opposition traded barbs on the future of Assad on Saturday.

Walid al-Muallem, Syrian foreign minister, said the government delegation in Geneva will reject any attempt by the UN envoy to include presidential elections on the agenda.

Syria refugees in despair, five years on

"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 said 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."

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.

They talks on Monday are part of the first diplomatic push since the Russian air force intervened in September to support Assad, tilting the war in his favour and helping the government reclaim significant areas in the west.

Syria's war: Past year described as the worst yet

On the ground, a Syrian military source told the Reuters news agency that rebel fighters targeted a warplane on Saturday while it was landing in Hama province, bringing it down, and calling the attack a breach of the  "cessation of hostilities" agreement.

There were conflicting accounts on Saturday night whether the government plane was brought down by missiles or anti-aircraft guns.

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 shot down the plane with two heat-seeking missiles.

The ceasefire agreement, brokered by the US and Russia, has been more widely respected than many expected, though fighting has continued on some important fronts, including near the Turkish border.

UpFront - Lakhdar Brahimi: 'No good guys in Syrian tragedy’

Source: Al Jazeera and agencies