By Tom Miles

GENEVA, March 14 (Reuters) - Syria faces a moment of truth, U.N. mediator Staffan de Mistura said on Monday as he opened the first of three rounds of peace talks envisaged to negotiate a "clear roadmap" for a future Syria.

Saying there was no "plan B" but a return to war, de Mistura asked to hear from all sides 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.

"If during these talks and in the next rounds we will 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 USA... and to the Security Council," he told a news conference.

The talks are the first to be held in more than two years and come amid an unprecedented cessation in hostilities sponsored by Washington and Moscow and accepted by President Bashar al-Assad's government and most of his foes.

The truce, the first of its kind in a 5 year-old war that has killed 250,000 people, has sharply reduced the fighting over the past two weeks, giving rise to hope that this diplomatic initiative will succeed where all previous efforts failed. The cessation was agreed after de Mistura called off a previous attempt to convene talks last month.

The talks must focus on political transition, which is the "mother of all issues", de Mistura said, while separate taskforces would keep tackling humanitarian issues and the cessation of hostilities.

"As far as I know, the only Plan B available is return to war, and to even worse war than we had so far."

All sides attending the talks have committed to a political transition that will follow the war, but Assad and his opponents disagree fundamentally on what that means, including whether the president must leave power.

The first round of talks will end around March 24, followed by a break of 7-10 days, then a second round of at least two weeks before another recess and a third round.

"By then we believe we should have at least a clear roadmap. I'm not saying agreement, but a clear roadmap because that's what Syria is expecting from all of us."

De Mistura did not mention whether Kurdish leaders would be involved for the first time, but said that the "proximity" format of indirect talks gave him flexibility to hear as many voices as possible, and all Syrians should be given a chance.

The main Kurdish YPG militia, which controls a swathe of northern Syria and is backed by the United States in combat with Islamic State fighters, has so far been excluded from talks in line with the views of Turkey, which considers it a terrorist group.

"The rule of the game will be inclusiveness," de Mistura said.

"In fact, the list of those whom we are going to consult or meet, or will be part of -- eventually, I hope -- not only of proximity negotiations but in fact direct negotiations is going to be constantly updated." (Reporting by Tom Miles; editing by Peter Graff)

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