United Nations-mediated peace talks between the Syrian government and opposition groups resumed Monday, with the UN special envoy calling them a "moment of truth" and insisting the "only Plan B available is return to war."

Staffan de Mistura warned that differences remained between the two sides, even as he praised the recent achievements on the ground in the war-weary country. The special envoy had suspended the talks only days after they started last month because of an upsurge in violence, and said failure this time could revive or even worsen the conflict, which marks its fifth anniversary on Tuesday.

"The alternative — some people call it `Plan B' as you know," he told reporters in Geneva. "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."

In the so-called proximity talks, the two sides don't meet face to face, but meet separately with de Mistura and his team. He then shuttles between them — in hopes of winning a breakthrough that could ultimately bring them to the table together, which is by most accounts a longshot as things now stand.

The talks began Monday with de Mistura hosting a government delegation led by Syria's UN ambassador, Bashar Jaafari.

Election call rejected

The two sides are deeply split on Syrian President Bashar Assad's future. 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 a presidential election 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, declining to comment on al-Moallem's remarks.

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 U.S. and Russia. Those two countries 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.

He said that if there doesn't appear to be a willingness to negotiate during these and future talks, "we will bring the issue back to those who have influence."

Conflict entering 6th year

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 extremist groups like IS and Nusra Front are excluded, and the cease-fire doesn't apply to areas they control.