The United Nations special envoy for Syria says the resumption of peace talks between Syrian government envoys and representatives of the opposition is a "moment of truth" for the war-torn nation.

Staffan de Mistura spoke to reporters on Monday moments before "proximity talks" were resumed in Geneva, a month after they were suspended amid an upsurge in violence in 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 United States and Russia, if the talks get bogged down.

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"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 told a news conference.

The talks came as he said a "fragile" ceasefire had largely held since February 27, and humanitarian aid deliveries had 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, but that the government and opposition were far apart on the future of President Bashar al-Assad.

Bays said the envoy was likely to proceed with an abundance of caution as talks opened.

"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.

"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."

TALK TO AL JAZEERA: Staffan de Mistura says no 'plan B' for Syria 

Syria's main opposition group, the High Negotiations Committee, or HNC, has announced it expects Geneva discussions to include Assad's departure and a timeframe for elections.

However, Bashar Jaafari, leading the government delegation to Switzerland, said in an interview with Syrian state TV late on Sunday that the opposition's discussions on removing Assad were an attempt to derail the peace talks before they had even begun.

As the negotiations opened on Monday, the UN agency for children highlighted the humanitarian crisis on the ground in Syria - saying 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 fighting has killed more than 250,000 people and displaced almost half the country's pre-war population of 23 million.

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Source: Al Jazeera and agencies