GENEVA: The UN special envoy for Syria says a resumption of peace talks between government envoys and representatives of the opposition is a "moment of truth" and insists the "only Plan B available is return to war."

Staffan de Mistura spoke to reporters Monday moments before resuming "proximity talks" in Geneva, a month after suspending them in an upsurge in violence in Syria. The talks come as he said a "fragile" cease-fire has largely held since Feb. 27, and humanitarian aid deliveries have resumed in recent weeks.

Failure in the talks could revive the war, de Mistura said.

"The alternative — some people call it 'Plan B' as you know," he said. "Well, as far as I know the only plan B available is return to war and to even worse war than we had so far."

The two sides deeply split on the future of President Bashar Assad. His foreign minister, Walid al-Moallem, said Saturday that any talk of removing Assad during a transitional period sought by the UN was "a red line," and rejected an international call for presidential elections to be held within 18 months — a key demand of the opposition.

"Spoilers will try to upset the talks ... public rhetoric will try to cast iron preconditions, but this is a moment of truth — and hopefully, proactive chance," de Mistura said.

De Mistura also said the onus for achieving successful talks rests with the UN Security Council, the International Syria Support Group of 18 regional nations and world powers — and especially the United States and Russia. Those two countries that have spearheaded the process and struck a deal that paved the way for the cessation of hostilities two weeks ago.

"The real peace-makers here are the peace-making powers who wanted these talks — the ISSG and the Security Council members — and hopefully the Syrian sides," de Mistura said. "If during these talks, and in the next rounds, we will see no notice of any willingness to negotiate, which we hope will not be the case, obviously we will do what we want to do, and what we have done: We will bring the issue back to those who have influence."

Western powers are largely backing the opposition — led by the Saudi-backed High Negotiations Committee — while Russia has been a pivotal supporter of Assad.

Many observers say the talks are the best chance in years to end a war moving into its sixth year, leaving at least 250,000 people dead, giving an opening to radical groups like Islamic State and the al-Qaida-backed Nusra Front to gain large swaths of territory, and forcing at least 11 million people to leave their homes, many abroad to places like Turkey, Jordan, Lebanon and Iraq, as well as to Europe.

Some call the talks "Geneva III" — a reference to two other rounds of talks that failed two years ago. UN-designated terror groups like IS and Nusra Front are excluded, and the cease-fire does not apply to areas they control.