Russia's intervention in the Syrian civil war has made the situation "much more dangerous", Defence Secretary Michael Fallon has said.
Russia says its air strikes, which are backed by Syrian President Bashar al-Assad, are aimed at so-called Islamic State and "other terrorists".
But the US and its allies say other rebel groups have been targeted.
Mr Fallon also said the government was "slowly building a consensus" among MPs for UK airstrikes over Syria.
He is to attend a Nato summit in Brussels on Thursday to discuss Russia's action and the fight against Islamic State (IS) extremists.