Syria's top government negotiator has described as "positive and constructive" his meeting with the UN special envoy for Syria after talks resumed in Geneva to end the conflict.
Bashar Jaafari, Syrian ambassador to the UN, said on Monday the government was interested in a dialogue that is "Syrian-led without foreign intervention and precondition".
Speaking to Al Jazeera from Geneva, Salim al-Muslet, of the opposition High Negotiations Committee (HNC), said his side was also "optimistic" but it insisted on a "political transition" in Syria "without [President Bashar] al-Assad."
"I believe that there are positive points that we can count on," he said.
"We are keen to find a solution that would lift the suffering of the Syrians.
"For Assad to be in power is not acceptable."
The HNC has said it expects the Geneva discussions to include Assad's departure and a timeframe for elections.
However, Jaafari, who is leading the government delegation, said the opposition's talk of removing Assad was an attempt to derail the peace talks before they could even begin.
He said "some" taking part in the talks "are trying to sabotage this round once again".
Earlier in on Monday, Staffan de Mistura, the UN special envoy for Syria, said the resumption of talks was a "moment of truth" for Syria.
Repeating his line that there is no "plan B" - other than a return to war - De Mistura asked to hear from all sides of the conflict but said he would have no hesitation in calling in the big powers, led by the US and Russia, if the talks get bogged down.
"If during these talks and in the next rounds we see no notice of any willingness to negotiate ... we will bring the issue back to those who have influence, and that is the Russian Federation, the US ... and the Security Council," he said.
Far apart on future
A "fragile" ceasefire has largely held since February 27, and humanitarian aid deliveries have resumed in recent weeks.
Al Jazeera's Diplomatic Editor James Bays, reporting from Geneva, said De Mistura wanted negotiations to focus on "substantive issues", including a new constitution and UN-monitored elections.
The government and opposition delegations were far apart on the future of Assad.
Our correspondent said De Mistura was likely to proceed with an abundance of caution.
"Mr De Mistura, throughout this, has been very keen to be positive and keep the momentum growing, because he knows there really are different crunch issues and if he was to dive into those straight away, the talks would collapse," he said.
TALK TO AL JAZEERA: Staffan de Mistura says no 'plan B' for Syria
"Yes, he says he wants substantive talks. Yes, he says he is going to deal with the mother of all issues - political transition - but he is going to do it very carefully."
As the negotiations opened on Monday, UNICEF, the UN agency for children, highlighted the humanitarian crisis on the ground in Syria, saying that more than 80 percent of Syria's children had been harmed by the five-year-old conflict.
UNICEF said a third of Syrians under the age of 18, or about 3.7 million, were born since the uprising against Assad erupted in 2011 and escalated into a full-blown civil war.
The Syrian conflict has killed more than 250,000 people and displaced almost half the country's pre-war population of 23 million.
Source: Al Jazeera and agencies