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 UN envoy said he hoped the move would help peace talks in Geneva.

As the first aircraft touched down in Russia, UN envoy Staffan de Mistura called President Vladimir Putin's move a "significant development" towards resolving a crisis which this week passes its fifth anniversary.

While Assad's opponents hope the announcement signals a shift in Russian support, its full significance has yet to become clear: Russia is keeping an airbase and an undeclared number of forces in Syria.

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

Putin's surprise announcement has led to speculation that Moscow may want to press Assad towards showing more flexibility at the Geneva talks, where his government has ruled out discussion of the presidency or a negotiated transfer of power.

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Russian warplanes have been flying home after President Vladimir Putin said his country would withdraw most of its forces from the country. (Vladimir Isachenkov/Associated Press)

Damascus has dismissed any talk of differences with its ally and says the move was coordinated and the result of army gains on the ground. Russia, however, last month expressed frustration with Assad, saying he was out of step with its diplomacy.

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," he said.

'Moment of truth'

The Geneva talks are part of a diplomatic push launched with U.S.-Russian support to end the war 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".

The opposition, which was due to meet de Mistura on Tuesday, said it was not against a direct meeting with the government.

U.S.-Russian cooperation had already brought about a lull in the war via a "cessation of hostilities agreement" that led to a sharp decline in fighting between rebels and the government in western Syria, though many violations have been reported.

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Putin, right, shakes hands with Syrian President Bashar al-Assad, left, in this Oct. 20, 2015 file photo. The Syrian presidency rejected speculation that the decision to remove troops reflected a rift between the allies. ( Alexei Druzhinin/RIA Novosti/Kremlin/Reuters)

The Syrian government, which had been losing ground 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".

A peace process for Syria endorsed by the UN 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.

The main opposition delegation 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.

Russian pilots welcomed home

Russian television showed the first group of Russia's Su-34 fighter plane task force returning from Syria and landing at an airbase in southern Russia.

The pilots were greeted by between 200 to 300 servicemen, journalists, and their wives and daughters, waving Russian flags, balloons in red white and blue, and flowers. Two priests also attended the event, parading a religious icon.

They were mobbed and thrown in the air by the crowd. A brass band played Soviet military songs and Russia's national anthem.

Their air campaign has mostly targeted insurgents battling Assad in western Syria, helping Damascus and allies including Hezbollah and Iranian Revolutionary Guards to reclaim important ground near the borders with Turkey and Jordan.

According to Russian Defence Minister Sergei Shoigu, Russia in the course of the operation flew more than 9,000 sorties. Military officials say they destroyed arms dumps, weapons, and fuel supplies being used by what they designated "terrorists".

The Syrian Organisation 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.

'A message to Assad'

Putin and U.S. President Barack Obama spoke by telephone on Monday about Syria, with the Kremlin saying the two "called for an intensification of the process for a political settlement".

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

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

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Russia says it will keep its advanced S-400 surface-to-air missile systems in Syria. (RIA Novosti/Reuters)

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."

Russia has previously called on Syria to prepare for presidential elections. The Syrian foreign minister said on Saturday these were the "exclusive right" of the Syrian people as he instructed his delegation to keep them off the agenda.

On the question of Assad, Moscow has said it is up to the Syrian people, not outside powers, to decide. Even Assad's enemies in the West have moved away from the demand he leave power right away.

Both Moscow and Tehran have an interest in his survival as a guarantor of both their interests.