Staffan de Mistura's dire assessment - and a rare admission of failure - came at the end of the eighth round of the largely ineffective, U.N.-sponsored talks in Switzerland. The U.N. envoy said he was "disappointed," after going in with ambitions to get the two sides to talk to each other.
De Mistura also offered some of his harshest criticism yet of the Syrian government delegation, accusing it of setting preconditions that he had warned against, and said its team refused to discuss anything other than "terrorism" - a term the government uses to refer to all armed opposition against President Bashar Assad.
The Damascus delegation also insisted the opposition team fell short of being an adequate representative of all of the opposition in Syria - a reference to groups that Assad's government has tolerated and that have not called for his ouster.
De Mistura said the opposition was engaged on other subjects, such as governance, the constitution and elections.
"Despite a lot of effort by my whole team, day and night with all sorts of creative formula, we did not have real negotiations," he said in frustration.
"It is a big missed opportunity, a golden opportunity missed at the end of this year," he said.
However, de Mistura, who has mediated the talks since early 2016, said he was still planning to reconvene the two sides next month - but will need to consult with U.N. Secretary-General Antonio Guterres first. Asked whether there was any point in returning to Geneva, de Mistura said the alternative would be abandoning Syria and a return to war.
"Our motto is never give up," he said.
The head of Syria's government team in Geneva, Bashar Ja'afari, extinguished any hopes of a last-minute breakthrough when he emerged earlier Thursday from a meeting with de Mistura to say his team would not talk to the opposition as long as it insists on Assad's removal from power.
In comments to reporters, he slammed the Syrian opposition for its so-called Riyadh communique, issued in Saudi Arabia before the latest round got underway in Geneva. The communique stated that the opposition's ultimate goal was to see Assad step down at the start of a political process.
The Damascus official described the statement as "blackmail of the Geneva process" and said its withdrawal was a precondition for talks. He also told reporters in Geneva that "those who drew up the Riyadh communique are the ones who sabotaged this round" of talks.
Nasr al-Hariri, the head of the Syria opposition delegation, warned that the Geneva talks are in "great danger" - accusing the government delegation of sabotaging them.
Al-Hariri said the "credibility of the international community is at stake" because Assad's government is "sabotaging the political process."
He noted that no date has been set for a next round of the Geneva talks, and said that under current circumstances "I don't think that another round would be quite effective."
"We call on the United nations and the U.N. special envoy to tell the entire world about the side that is undermining the political process and to sabotage the Geneva process," he told reporters Thursday evening.
Earlier, de Mistura called on Russian President Vladimir Putin to "have the courage" to push his Syrian government allies to accept new elections and a new constitution.
In an unusual public appeal directly to a key power broker in the region, de Mistura told a TV interviewer the Russian leader, Vladimir Putin, should "convince the (Syrian) government that there is no time to lose" in efforts to reach peace after nearly seven years of war.
Russia has provided crucial military and diplomatic backing to Assad's forces, though Putin announced a drawdown of his forces in Syria during a surprise visit to the Mideast country on Monday. The Russian president touched down at a Russian military base in Syria in the wake of battlefield successes to laud the troops' achievements and tell them that a significant number would be going home.
Asked what signal Putin could provide now, de Mistura said territorial gains are "temporary."
"Peace must be won - and for the peace to be won, it's necessary to have the courage to push the government also to accept that there must be a new constitution and new elections," the U.N. envoy added in comments late Wednesday to Swiss broadcaster RTS.
Ja'afari also blasted de Mistura for his comments on Russia, saying such positions "derail his mandate as a facilitator of the talks, which will have an impact on the Geneva process all together.
In the interview, de Mistura said it was "regrettable" that Assad's delegation had refused to meet face-to-face with the opposition. At the start of the latest round, the delegations at one point were just meters (yards) away from one another in separate rooms.
With Russia pushing a separate peace talks track, de Mistura emphasized the importance of a U.N. role in any peace process.
Hinting at his repeated concerns about a de facto partition of Syria, the U.N. envoy held up a color-coded map showing the divisions of territorial control in Syria.
The war is estimated to have killed at least 400,000 people and driven over 12 million from their homes. De Mistura's two predecessors stepped down in frustration after failing to jumpstart peace talks.
Karam reported from Beirut.
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