Russia will be judged by its actions following Vladimir Putin's surprise announcement that he will pull troops out of Syria, Foreign Secretary Philip Hammond said.
There was no independent evidence that the withdrawal had begun, Mr Hammond said, and it was "not terribly reassuring" that Russia's defence ministry had indicated air strikes could continue against "terrorists" - a term Moscow has used to include groups the West views as moderate opposition forces.
The Foreign Secretary sounded a cautious note, telling MPs that Russia had previously claimed it was withdrawing from the Ukraine when it was actually just a routine rotation of troops.
It was unclear what Mr Putin's intentions were as he is a "very difficult partner", the Foreign Secretary added.
Prime Minister David Cameron's official spokeswoman said that the withdrawal would be "welcome" if it signalled a new commitment to a genuine political transition in the war-torn Middle Eastern state.
Mr Cameron spoke with Mr Putin about Syria less than a fortnight ago, in a conference call with other European leaders, but Number 10 left no doubt that the Russian president's decision to pull troops out took Britain by surprise.
Updating MPs on the situation, Mr Hammond appeared to acknowledge that Moscow's move had caught out the international community, with no members of the International Syria Support Group given advance notice of Mr Putin's intentions.
Mr Cameron had not yet spoken to Mr Putin since his announcement on Monday night that Russia would be withdrawing "the main part" of its forces, including some aircraft, from Syria.
Russia's defence ministry announced early on Tuesday that the first group of warplanes had left their air base in Syria although Mr Hammond said there "we do not yet have any independent evidence to verify Russia's claims that military withdrawals have already begun".
Moscow is still expected to maintain a naval base and air base, as well as some troops in the country.
Russia's intervention, which took the world by surprise when air strikes began in September, has tipped the military balance in favour of Moscow's protege Bashar Assad, with Western nations accusing Mr Putin of targeting not only the terrorists of Islamic State - also known as Daesh - but also more moderate groups opposed to the Damascus regime.
Moscow has suggested the Syrian president agreed with the decision to pull the bulk of Russian troops out.
Mr Putins's declaration that action to shore up Assad had been successful was seemingly timed to coincide with a fresh round of peace talks in Geneva led by United Nations envoy Staffan de Mistura.
Mr Hammond said: "No one would be more delighted than me if, after five months of relentless bombing, Russia is genuinely winding down its military support to the brutal Assad regime.
"But, as in all matters related to Russia, it is the actions rather than the words that count. We shall be watching carefully over the coming days to see if the potential promise of this announcement turns into reality."
He told the Commons it was "worth remembering" that Russia had previously announced the withdrawal of military assets from Ukraine "which later turned out merely to be routine rotation of forces".
Mr Hammond stressed the UK's stance remained that "there can be no peace in Syria while Assad remains in power" and Moscow must use its influence to advance the Geneva process following the partial ceasefire announced in February.
"We look to Russia as guarantor for the regime and its backers to use its unique influence to ensure compliance and to make very clear to the Assad regime that their expectation is that they must negotiate in good faith," he said.
"After investing so much in Assad, Mr Putin must show the world that he can exercise control over his protege."
Mr Hammond also expressed concern about reported remarks made by Russian defence minister Sergey Shoygu about continuing to strike terrorists "exactly the formula that the Russians have used in the past when attacking the moderate opposition".
"It's not terribly reassuring that a few hours after the announcement of a withdrawal of their military forces their defence minister is saying that they will continue to attack terrorists."
The Foreign Secretary said it was possible the Russian announcement was "intended as a message" to the Assad regime not to "overplay your hand" and to engage in negotiations, or a signal to the opposition groups "because it has not been that easy to persuade them to attend the Geneva talks when Russian bombs are still raining down on their positions".
"But 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."
Tuesday marks the fifth anniversary of a first wave of anti-Assad protests widely held to have signalled the start of the Syrian uprising.
The ensuing civil war has since claimed more than 250,000 lives, displaced half of Syria's population and flooded Europe with refugees. There have also been a number of British citizens who have travelled to the region to fight for terror groups including Islamic State.
US president Barack Obama spoke with the Russian leader on Monday about the next steps and ned "some progress" in providing access to humanitarian aid for Syrian civilians, the White House said.