Syria conflict: Kerry plans key talks in Russia
US Secretary of State John Kerry will visit Russia next week to discuss Syria, after Moscow began withdrawing most of its forces from the country.
Mr Kerry said Russia's move, along with the opening of UN-mediated Syria talks in Geneva, may be "the best opportunity" to end the conflict.
The UN special envoy to Syria said the talks now had a new momentum.
The first Russian planes started leaving on Tuesday, a day after Russia's announcement.
However, Russia would continue its air strikes in Syria despite the partial pullout, Deputy Defence Minister Nikolay Pankov said.
"Today, as we mark the fifth anniversary of the start of this horrific war, we may face the best opportunity that we have had in years to end it," Mr Kerry said on Tuesday.
He spoke of an "important phase" in the Geneva talks.
Mr Kerry is expected to go to Moscow some time after Tuesday when he returns from a trip to Cuba.
Meanwhile, the UN Special Envoy to Syria, Staffan de Mistura, who is mediating in the Geneva talks, welcomed the Russian decision.
"The announcement by President [Vladimir] 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.
More on the Syria conflict
- IS: The full story
- Why has fighting gone on for five years?
- How Putin took command in Syria
- Russia's 'peace mission' in Syria
- Tide turns Assad's way
- Russian media hail withdrawal
- Russian pullback: Syrians react with dark humour
- Russian withdrawal a catalyst for peace?
Russian TV showed planes on Tuesday arriving from Syria 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.
The war in Syria has raged for five years and claimed the lives of more than a quarter of a million people. Millions have fled the conflict, but nearly 18 million people still live in the war-torn country - so what is life like for them?
War crimes report
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.
The Russian air campaign started last September, tipping the balance in favour of the Syrian government and allowing it to recapture territory from rebels.
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.
A cessation of hostilities involving most participants has been largely holding despite reports of some violations.
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.
Inquiry 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".
Why did Russia launch an air campaign in Syria?
Russia is one of President Bashar al-Assad's most important international backers and the survival of his government is critical to maintaining Russian interests in Syria. Russia has a key naval facility which it leases at the port of Tartous and has forces at the Hmeimim airbase in Latakia.
In September 2015, with rebel forces advancing on Latakia, Russian forces launched an air campaign which President Vladimir Putin said was aimed at "stabilising" the Syrian government and creating conditions for "a political compromise" that would end the five-year conflict.
What does Russia say its intervention achieved?
In March 2016, Mr Putin ordered the "main part" of Russia's forces to withdraw from Syria, saying their mission had "on the whole" been accomplished.
Defence Minister Sergei Shoigu said Russian aircraft had flown more than 9,000 sorties over almost six months, killing more than 2,000 "bandits" and helping Syrian government forces regain control of 10,000 sq km (3,860 sq miles) of territory, including 400 population centres.
The claims have not yet been independently verified, but it is clear the air campaign turned the tide of the war in favour of Mr Assad, allowing Syrian government ground forces to regain territory around Latakia, in the southern province of Deraa and around the divided northern city of Aleppo.
What do critics say?
Moscow stressed that its air strikes only targeted "terrorists", but activists said Russian aircraft had mainly bombed Western-backed rebel groups and civilian areas.
In December, Amnesty International said Russian aircraft appeared to have directly attacked civilians by striking residential areas with no evident military target, which it warned might amount to war crimes. Russia's defence ministry dismissed the report as containing "fake information".
However, the UK-based Syrian Observatory for Human Rights reported in early March that 1,733 civilians, including 429 children, had been killed in Russian air strikes, along with some 1,492 rebels and members of the al-Qaeda-affiliated al-Nusra Front, and 1,183 Islamic State (IS) militants.