The first Russian planes flew out of Syria on Tuesday, at the start of a surprise withdrawal diplomats are hoping will boost a new round of UN-backed peace talks.

A group of Su-34 bombers and a Tu-154 transport plane left Moscow's Hmeimim base in Syria headed for home, the defence ministry said, a day after President Vladimir Putin ordered most of his forces out of the war-torn nation.

Putin on Monday said Moscow's military goal had been "on the whole" completed some five-and-a-half months and 9000 combat sorties after the Kremlin launched its bombing campaign in support of long-time ally President Bashar al-Assad.

Western leaders reacted cautiously to the announcement, with Moscow yet to specify a timeframe for the withdrawal and set to maintain its air and naval base and advanced air defence systems in Syria.

There have also been reports Russia intends to continue air strikes.

"Certain positive results have been achieved... However, it is too early to talk about victory over terrorism," Deputy Defence Minister Nikolay Pankov said, Ria news agency reports.

"A Russian air group has the task of continuing to strike terrorist facilities."

UN peace envoy Staffan de Mistura nevertheless described the Russian pull out as a "significant development" for the talks that began in Geneva on Monday.

“We hope will have a positive impact on the progress of the negotiations," he said in a statement.

Hopes for a breakthrough at the Geneva talks however remained remote, with both sides locked in a bitter dispute over Assad's future as the conflict in Syria entered its sixth year.

As the talks entered their second day, Mr De Mistura was expected to hold his first official meeting with the Syrian opposition High Negotiations Committee (HNC), which has repeatedly said that Assad cannot be part of Syria's political future.

But the withdrawal of the Russian troops – which began airstrikes in support of the regime in September, sparking condemnation from Western powers – is expected to put more pressure on Assad to negotiate during the Geneva talks.

"If the announcement of a withdrawal of Russian troops materialises, this increases the pressure on President Assad to finally negotiate in a serious way in Geneva a political transition which maintains the stability of the Syrian state and the interests of all populations," German Foreign Minister Frank-Walter Steinmeier said.

The Russian ambassador to the United Nations Vitaly Churkin also said the Kremlin's move would boost chances of a diplomatic solution to a conflict that has killed more than 270,000 people and displaced millions.

Russia began its airstrikes in support of Assad's army in September, a move that helped shore up the regime's crumbling forces and allow them to go on the offensive.

Russia sent some 50 warplanes to carry out thousands of strikes across Syria arguing that it was targeting "terrorist" groups including Islamic State jihadists.

The intervention was slammed by the West and its regional allies, who insisted that Moscow was mainly bombing more moderate rebels fighting Assad.

A temporary ceasefire between Assad's forces and opponents in the country introduced on February 27 has largely held, but does not cover the IS and Al-Nusra Front groups.