Russia's military build-up in Syria is further complicating efforts to resolve the country's bitter civil war, Defence Secretary Michael Fallon has warned.
Mr Fallon said that, unlike the Americans, Britain had not had any discussions with Moscow about military operations against Islamic State (IS) militants - also referred to as Isil - in Syria.
His comments came amid reports that Russia has deployed warplanes, helicopters, tanks, artillery, armoured personnel carriers and 200 marines to an airfield near President Bashar al-Assad's ancestral home in Latakia province.
Speaking at the Royal United Services Institute in London, Mr Fallon said that while there was an urgent need to end the conflict, Russia's efforts to bolster the Assad regime were making it more difficult.
"The Russian action in the last few weeks putting ships and aircraft into the region further complicates an immensely complicated situation," he said during a question and answer session.
"Thousands of people have already been killed, millions are being displaced. You have a regime that is barrel-bombing its own citizens and you have Isil with its own brand of barbarity.
"So it is urgent that we find a way through to bring peace to that particular country. The Russian intervention certainly makes the situation even more complicated."
Following the election of new Labour leader Jeremy Corbyn - who opposes UK military intervention - Mr Fallon indicated the Government would need the support of rebel Labour MPs if it was to win a Commons vote on extending RAF airstrikes against IS in Iraq into neighbouring Syria.
"The Prime Minister has made it very clear that to extend our operations into Syria we need the support of Parliament and we will not go and ask for that support until we are sure of getting it. We will have to make the argument on both sides of the House," he said.