MOSCOW/BEIRUT/GENEVA (Reuters) - Russian warplanes flew home from Syria on Tuesday as Moscow started to withdraw forces that have tipped the war President Bashar al-Assad’s way, and the U.N. envoy said he hoped the move would help peace talks in Geneva.

As the first aircraft touched down in Russia, U.N. envoy Staffan de Mistura called President Vladimir Putin’s surprise move a “significant development” toward resolving a conflict which this week passes its fifth anniversary.

Assad’s opponents hope Putin’s announcement on Monday that most Russian forces would be withdrawn signaled a shift in his support. However, its full significance is not yet clear: Russia is keeping an air base and undeclared number of forces in Syria.

Russian jets were in action against Islamic State on Tuesday. Assad also still enjoys military backing from Iran, which has sent forces to Syria along with Lebanon’s Hezbollah.

Russia said last month Assad was out of step with its diplomacy, prompting speculation Putin is pushing him to be more flexible at the Geneva talks, where his government has ruled out discussion of the presidency or a negotiated transfer of power.

Damascus has dismissed any talk of differences with its ally and says the planned withdrawal was coordinated and the result of army gains on the ground.

British Foreign Secretary Philip Hammond, whose government supports the opposition, indicated the gaps in Western understanding of Putin, saying he had “no insight at all into Russia’s strategy” after a decision that came out of the blue.

The West had been equally surprised by Putin’s decision to intervene. “Unfortunately none of us knows what the intent of Mr Putin is when he carries out any action, which is why he is a very difficult partner in any situation like this,” Hammond said.

Analysts in Moscow said Putin’s acquisition of a seat at the diplomatic top table may have motivated his move to scale back his costly Syria campaign.

“MOMENT OF TRUTH”

Russia appeared to be following through on its pledge, the U.S. White House said, but spokesman Josh Earnest said it was too early to assess the broader implications, adding Moscow did not give the United States direct notice of its withdrawal plan.

U.S. Secretary of State John Kerry welcomed Putin’s announcement and said he planned to visit Moscow next week for what he called the best opportunity in years to end the war.

The Geneva talks are part of a diplomatic push launched with U.S.-Russian support to end a conflict that has killed more than 250,000 people, created the world’s worst refugee crisis, and allowed for the rise of Islamic State. Opening the indirect talks, de Mistura said Syria faced a “moment of truth”.

U.S.-Russian cooperation has already brought about a lull in the war via a “cessation of hostilities agreement”, though many violations have been reported.

Opposition negotiators demanded on Tuesday that the government spell out its thoughts about a political transition in Syria, saying there had been no progress on freeing detainees, who were being executed at a rate of 50 a day.

The opposition High Negotiations Committee (HNC) used their first meeting in the round of peace talks to give de Mistura a set of general principles to guide the transition.

A peace process for Syria endorsed by the U.N. Security Council in December calls for a Syrian-led process that establishes “credible, inclusive and non-sectarian governance”, a new constitution, and free, fair elections within 18 months.

Slideshow ( 16 images )

The HNC wants Assad out of power by the start of a transition. While some rebels have expressed guarded optimism at Putin’s announcement, others doubt he is about to put serious pressure on Assad.

“We do not trust them,” said Fadi Ahmad of the First Coastal Division, who says his rebel group has been fighting a Russian-backed government offensive near the Turkish border throughout the cessation agreement that came into effect on Feb. 27.

The Syrian government, which had been losing territory to rebels before Russia intervened, had indicated it was in no mood to give ground to the opposition on the eve of the talks that started on Monday, calling the presidency a “red line”.

Slideshow ( 16 images )

PILOTS WELCOMED, RUSSIAN JETS STAGE STRIKES

Russian television showed the first group of Su-34 jets landing from Syria at a base in the south of the country.

The pilots were greeted by 200-300 servicemen, journalists, and their wives and daughters, waving Russian flags, balloons in red white and blue, and flowers. They were mobbed and thrown in the air by the crowd. A brass band played Soviet military songs and the national anthem. Two priests paraded a religious icon.

Russia flew more than 9,000 sorties during the Syrian operation, according to Defence Minister Sergei Shoigu. Military officials say they destroyed arms dumps, weapons and fuel supplies being used by what they called terrorists.

The Syrian Observatory for Human Rights, which reports on the war using sources on the ground, says Russian air strikes have killed more than 1,700 civilians. Moscow denies that.

Showing Russian warplanes were still active in Syria, heavy air support was reported helping the Syrian army make major gains against Islamic State near the ancient city of Palmyra. IS is not included in the cessation of hostilities.

At least 26 people were killed east of the Islamic State-held city on Tuesday, the British-based Observatory reported.

“MESSAGE TO ASSAD”

Putin said Russia had largely fulfilled its objectives in a campaign which has so far cost Russia $700-$800 million according to a Reuters estimate, an additional financial burden at a time of low oil prices.

Russia, which has haunting memories of the long Soviet occupation of Afghanistan, said it would be keeping its most advanced air defense system, the S-400, in Syria.

A Western diplomat said Putin would “now move to focus on the peace talks and this will put pressure on the Syrian government to negotiate”. The diplomat added: “We don’t know if he is giving up on Assad but we know that the Russians are delivering a message to Assad that they are keen on negotiations over transition to proceed.”

Moscow has said it is up to the Syrian people, not outside powers, to decide Assad’s future. Even Assad’s enemies in the West have moved away from demanding he leave power immediately.

In Geneva, U.N. war crimes investigators on Syria said lower-level perpetrators should be prosecuted by foreign authorities until senior military and political figures can be brought before international justice.

The U.N. Commission of Inquiry, which has documented atrocities by all sides, has compiled a confidential list of suspects and maintains a database with 5,000 interviews.

“The adoption of measures that lay the ground for accountability need not and should not wait for a final peace agreement to be reached,” Paulo Pinheiro, chief of the inquiry panel, told the U.N. Human Rights Council.