GENEVA (AP) — The eighth round of Syria peace talks got under way on Tuesday with a revamped and expanded opposition team meeting with the U.N. envoy in Geneva, but President Bashar Assad's government was conspicuously absent and Russia bemoaned back-channel diplomacy among key world powers.

The government in Damascus said its team would show up on Wednesday.

The meeting marks the latest and perhaps most-promising attempt by U.N. Syria envoy Staffan de Mistura to jolt new life into the so-far largely ineffective U.N.-mediated talks. The Syrian civil war, now in its seventh year, has left over 400,000 dead, driven at least 11 million people from their homes, and turned into a proxy war for regional and world powers.

Speaking after he briefed key envoys from the five U.N Security Council member countries behind closed doors, de Mistura confirmed to The Associated Press that the Syrian government had agreed to come Wednesday — after Damascus had reportedly delayed its initial participation.

The pro-government newspaper Al-Watan on Monday reported that Damascus would postpone its arrival, citing the opposition's position that Assad must leave at the start of any political transition — something that is sought under a key U.N. Security Council resolution underpinning the Geneva talks.

De Mistura justified the discreet "P5" meeting by saying it was needed to keep Britain, China France, Russia and the United States informed about his agenda and reinforce the U.N.-mediated peace process, which has come under a challenge from a separate, Russian initiative.

"Now we need to go into business," de Mistura told the AP. "We need P5 support because the two delegations obviously listen to members of the P5."

The meeting included Brett McGurk, the U.S. envoy for the battle against the Islamic State group, and French Foreign Ministry political and security affairs director Nicolas de Riviere — among the highest-level Western envoys on hand so far during Geneva talks.

Russia's ambassador in Geneva, Alexey Borodavkin, was less positive about the back-channel briefing. He said Russia appreciated the "opportunity," but "at the same time, we have some doubts about this kind of format, and do not consider it an established one."

Moscow has sought to take the lead in resolving Syria's conflict by organizing parallel talks between the two sides in the Black Sea resort of Sochi. But those efforts have sputtered as Western powers and de Mistura insist Geneva must remain the primary track for any negotiations.

The Syrian government, with pivotal military support from by Russian and Iranian forces, has made key gains on the ground over the last year.

One of the major questions is whether Assad's envoys will finally sit down face-to-face with the opposition and negotiate.

The unexplained tardiness of Assad's delegation in Geneva appeared to reflect the government's displeasure with the opposition's insistence that Assad must step down at the start of an eventual transitional period in Syria. The opposition says it has no preconditions to the talks, but at the same time says its goal remains to push the longtime leader aside.

As in previous rounds of the U.N.-sponsored talks, discussing Assad's future will likely be the main obstacle to any progress in Geneva. Damascus has refused to negotiate Assad's future with the opposition and says it wants instead to focus on defeating "terrorism" — the government's byword for armed opponents of the Syrian president.

The chief of the opposition's delegation, Nasr Hariri, told reporters Monday that the opposition was "ready to negotiate," and accused the government of stalling.

"The thing the regime is most afraid of is political negotiations," Hariri said.

The opposition had said it would enter into talks "without preconditions" but maintained its position that Assad would need to go at the start of any transitional period for any settlement to the civil war to succeed.

The opposition delegation was expanded last week under Saudi Arabian auspices to include groups seen by Damascus as more palatable for negotiations, including the so-called "Moscow group" which has resisted calling for Assad's departure.

Hariri said the reformulation of the opposition team in Geneva removed any excuse for the government and its chief diplomatic backer, Russia, to circumvent the U.N. talks.

De Mistura stressed he would "not accept any preconditions by any party", and said the talks would be guided by a 2015 Security Council resolution mandating a political transition for Syria. He wants an "inclusive process" that can ultimately draft and ratify a new constitution.

U.S. and British officials expressed support for the Syrian opposition ahead of the talks Tuesday.

Alastair Burt, the British minister of state for the Middle East, said after meeting Hariri that the talks need "to lead toward the inclusive political transition necessary to end this conflict." Hariri also met with U.S. Deputy Assistant Secretary of State David Satterfield, who offered similar support.

A Syrian foreign ministry official, who was not identified under Syrian government regulations, said its delegation will part starting Wednesday afternoon. The Damascus team will again be headed by Bashar Ja'afari, Syria's representative at the United Nations in New York, the official said.

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Aji reported from Damascus, Syria. Associated Press Writer Philip Issa in Beirut contributed to this report.