Russia will continue air strikes in Syria despite the withdrawal of most of its forces, a senior official has said.

Deputy Defence Minister Nikolay Pankov said it was too early to speak of defeating terrorism, after a campaign that has bolstered Syria's government.

Russian forces started leaving Syria on Tuesday after Monday's surprise announcement by President Vladimir Putin. Some have now landed in Russia.

A second day of peace talks is being held aimed at resolving the conflict.

UN Special Envoy Staffan de Mistura, who is mediating in the talks, welcomed the Russian decision.

"The announcement by President Putin on the very day of the beginning of this round of Intra-Syrian Talks in Geneva is a significant development, which we hope will have a positive impact on the progress of the negotiations," he said.

As the pullout got under way, Russian air strikes were reported against so-called Islamic State (IS) militants near the IS-held city of Palmyra.

Lebanon's al-Manar TV and the UK-based Syrian Observatory for Human Rights said Syrian forces, supported by Russian air cover, had advanced towards the city.

IS captured Palmyra last May, deliberately destroying parts of the Unesco World Heritage site in scenes which caused outrage around the world.

Russian defence ministry video showed the first group of aircraft taking off from Hmeimim air base in Syria on Tuesday morning and in flight.

Hours later, Russian TV showed planes arriving in the southern Russian city of Voronezh, where they were greeted on the tarmac by priests and crowds waving balloons.

Su-24 tactical bombers, Su-25 attack fighters, Su-34 strike fighters and helicopters were returning home, the TV said.

But Mr Pankov said a Russian air group would remain.

"Certain positive results have been achieved... However, it is too early to talk about victory over terrorism. A Russian air group has the task of continuing to strike terrorist facilities," he was quoted by Ria news agency as saying.

Another senior official, upper house defence committee head Viktor Ozerov, said as many as two battalions - some 800 servicemen - could remain in Syria after the withdrawal to guard Hmeimim and the naval base at Tartous, Interfax news agency reported.

Military advisers training Syrian government troops would also stay, he added.

Meanwhile Kremlin chief-of-staff Sergey Ivanov said Russia would keep its advanced S-400 surface-to-air missile system in place.

"We are leaving completely reliable cover for the remaining contingent... To effectively ensure security, including from the air, we need the most modern air defence systems," Russian media quoted him as saying.

The Russian force reduction was announced during a meeting on Monday between Mr Putin and his defence and foreign ministers.

Russia is a key ally of Syrian President Bashar al-Assad, and his office sought to reject speculation there was a rift between the two countries, saying the move was mutually agreed.

It has received a guarded welcome from Western diplomats and the Syrian opposition.

War crimes report

The Russian air campaign started last September, tipping the balance in favour of the Syrian government and allowing it to recapture territory from rebels, but on Tuesday the defence ministry announced the withdrawal.

It is not clear how many military personnel Russia has deployed, but US estimates suggest the number ranges from 3,000 to 6,000, AP reports.

Russia had long insisted its bombing campaign only targeted terrorist groups but Western powers had complained the raids hit political opponents of President Assad.

In a statement, the Syrian government said the plan was agreed between the two countries.

Most participants in the Syria conflict agreed to a cessation of hostilities, which has been largely holding despite reports of some violations on all sides.

Meanwhile, the UN Commission of Inquiry on Syria has presented its report on war crimes committed by all sides in Syria's war to the UN Human Rights Council in Geneva.

Its chairman Paulo Pinheiro said the task of pursuing war criminals should not wait for a final peace agreement as there was now "hope of an end in sight".