The Government remains committed to building parliamentary support for RAF air strikes against Islamic State (IS) in Syria despite Russia's intervention in the conflict, Defence Secretary Michael Fallon has said.
Following talks in London with US defence secretary Ash Carter, Mr Fallon said Russian air and missile strikes in support of president Bashar Assad risked making "a grave situation even worse".
However he made clear that ministers would not be deterred from seeking support for a new Commons vote for British warplanes to join the US and other coalition aircraft in attacking IS - also referred to as Isil - in Syria as well as Iraq.
"We will continue to build the case for military action in north-east Syria where Isil is headquartered, where its command and control is," he said.
"Russian intervention doesn't change that. We will continue to build the case for intervention in the new parliament."
His comments came after the former chief of MI6, Sir John Sawers, warned that there was a "real danger" of clashes between Russian and coalition warplanes if they continued to mount rival operations in the skies over Syria.
David Cameron has repeatedly made clear that while he wants to join the US in attacking IS in Syria, he will only seek a Commons vote if he can be sure of victory, which would require the support of a significant number of Labour MPs.
The talks between Mr Carter and Mr Fallon came after Nato defence ministers agreed on Thursday to a doubling of the alliance's response force - to 40,000 - amid fears from eastern European states that they could be the next target of Russian aggression.
Britain is to deploy around 100 troops to the Baltic states as part of a new US/German-led training, evaluation and capacity-building mission intended to strengthen Nato's "persistent presence" in the region.
Mr Carter said president Vladimir Putin's support for the Assad regime was "illogical" and would ultimately only succeed in increasing support for the militants while turning the world against Moscow.
"They are going to have the effect of inflaming the very extremism that Russia claims to want to combat," he said.
"By taking the side of Assad they inflame the civil war - and therefore extremism - and prolong the suffering of the Syrian people.
"They are going to have the effect also of turning everyone against Russia itself. This will boomerang in a very direct way on Russia's security."
Mr Fallon condemned the Russians for attacking mainstream opposition groups as well as the extremists who they claimed to be targeting.
"That is very regrettable, in particular the mounting toll of civilian casualties through what looks like the use of unguided as well as guided munitions being deployed quite indiscriminately in civilian areas," he said.
Mr Carter declined to expand on claims by US officials - denied by Moscow - that four cruise missiles launched by Russian warships in the Caspian Sea had crashed in Russia's ally, Iran.
"We did have some indications that was the case. If so, that would indicate malfunctions of those missiles," he said.