The United States and France have accused the Syrian government of trying to disrupt the upcoming peace talks in Geneva and criticised its new conditions.
Syrian opposition negotiators have started arriving in the Swiss city in advance of the talks, which are expected to tackle the issue of President Bashar al-Assad's presidency.
However, the Syrian government delegation, which has already arrived in Geneva, insists that removing Assad is a "red line".
Speaking after talks with European allies in Paris on Sunday, John Kerry, the US secretary of state, said the comments were a provocation and that Russia and Iran would need to show the Syrian government was "living up to" what had been agreed.
Kerry said the Syrian government and its backers were mistaken if they thought they could continue to test the boundaries of a fragile truce.
Accusing Syria of carrying out the most violations of the truce, he said Russian President Vladimir Putin needed to look at how Assad was acting.
"President Assad is singing on a completely different song sheet and sent his foreign minister out yesterday to try to act as a spoiler and take off the table what President Putin and the Iranians have agreed to," Kerry said.
For his part, Jean-Marc Ayrault, the French foreign minister, said: "It's a provocation ... a bad sign and doesn't correspond to the spirit of the ceasefire."
The Syrian government said on Saturday that it would not discuss presidential elections at the Geneva conference or hold talks with any party wishing to discuss the question of the presidency.
Walid al-Muallem, Syria's foreign minister, said the government delegation will reject any attempt by the UN envoy 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 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."
Monday's 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.
Al Jazeera's Diplomatic Editor James Bays, reporting from Geneva, said Staffan de Mistura, the UN envoy, has openly stated that 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.
Our diplomatic editor said: "I think what is important this time around [compared with 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."
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 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.
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 as to 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.
Source: Al Jazeera and agencies