MOSCOW: Russia on Friday welcomed Syrian President Bashar al-Assad's "decisive" victory after he claimed re-election with a landslide win in a vote criticised by the opposition and Western nations.

"A decisive victory was won by the incumbent head of state," the foreign ministry said in a statement.

"We view the elections as a sovereign affair of the Syrian Arab Republic and an important step towards strengthening its internal stability," it added.

The controversial vote extending Assad's stranglehold on power was the second since the start of a decade-long civil conflict that has killed more than 388,000 people, displaced millions and battered the country's infrastructure.

On the eve of the election, the United States, Britain, France, Germany and Italy said the poll was "neither free nor fair", and Syria's fragmented opposition has called it a "farce".

Russia has been a key ally to the Syrian regime in the conflict, and its intervention in the war in September 2015 was seen as having turned the tide of the fighting in Assad's favour.

The foreign ministry described Western statements calling into question the validity of the elections as "another attempt to interfere in the internal affairs of Syria with the aim of destabilising it".

"No one has the right to dictate to the Syrians when and under what conditions they should elect their head of state," it said.