United Nations-mediated peace talks have started in Geneva between Syrian President Bashar al-Assad's regime and representatives of Syrian opposition factions.
The talks on March 14 are aimed at capitalizing on a shaky truce between Syrian government forces and opposition fighters.
The UN special envoy for Syria, Staffan de Mistura, said a resumption of peace talks is a "moment of truth" and insisted the "only Plan B available is return to war."
De Mistura, speaking to reporters moments before resuming talks in Geneva, added that agreeing on political transition in Syria is the "mother of all issues."
The long-awaited talks, which open on the eve of the fifth anniversary of the outbreak of the conflict, mark the latest bid to end the bloodshed that has killed more than 270,000 people and displaced millions.
De Mistura said that he expected the talks set to begin on March 14 would be the first of three rounds.
The current one is expected to last until March 24, with a second round beginning after a recess of a week or 10 days, with that round lasting "at least two weeks."
He said a third round would be held after another recess.
Russia again called for all opposition factions to be represented at the Syrian peace talks.
"It's clear that they should include the whole spectrum of Syrian political forces, otherwise this cannot claim to be a representative forum," Foreign Minister Sergei Lavrov said in Moscow on March 14.
Moscow has called on de Mistura to include Syrian Kurds in the talks. De Mistura has said that while Syrian Kurds would not take part, they should be given a chance to express their views.
Lavrov also said Russia would support whatever solution the Syrian government and the opposition devise to end the country's war, including "any form [of government] whatever it may be called: federalization, decentralization, unitary state."
An attempt in early February to bring together representatives of Assad's regime and moderate opposition factions that are grouped together in the High Negotiations Committee broke down when opposition delegates refused to attend sessions in the midst of a Russian-backed Syrian government offensive against opposition fighters at Aleppo.
The March 14 talks in Geneva were arranged after the truce deal was brokered by Russia and the United States.
On March 13, U.S. Secretary of State John Kerry warned Assad's regime and its allies against exploiting the truce deal.
He was speaking after talks in Paris with EU foreign-policy chief Federica Mogherini and the foreign ministers of France, Britain, Germany, and Italy.
The negotiations in Geneva are set to cover the formation of a new Syrian government, a fresh constitution, and the organization of UN-monitored presidential and parliamentary elections within 18 months.
The High Negotiations Committee, a Saudi-backed umbrella opposition group, said it will attend the Geneva talks and press for a transitional government with full executive powers that does not include Assad or any of his close associates.
Opposition negotiator Muhammad Allush -- who also is the leader of a group called Jaysh al-Islam (Army of Islam) -- said in Geneva on March 12 that "the transitional period should start with the fall, or death, of Bashar al-Assad."
Allush said a transitional government "cannot start with the presence of the regime, or the head of this regime still in power."
Bashar al-Jaafari, the head of the Syrian negotiation team in Geneva, accused Allush of being a Saudi-backed "terrorst."
Jaafari said on March 13 that Allush's call for "the fall, or death, of Bashar al-Assad" was, in Jaafari's words, "a clear attempt to ruin this round" of peace talks and the efforts of the UN envoy on Syria.
Syrian Foreign Minister Walid al-Muallam said opposition calls for Assad's removal from power would be a "red line" and suggested that it would cause the negotiations in Geneva to break down.
Muallam said: "We will not talk with anyone who wants to discuss the presidency.... Bashar al-Assad is a red line. If they continue with this approach, there's no reason for them to come to Geneva."