British troops are to be deployed to the Baltics as part of a long-term Nato mission to deter Russian aggression.
Defence Secretary Michael Fallon said a "small number" of soldiers would be stationed there to "watch over our allies on the eastern borders".
RAF Typhoon fighters are already in the region as part of the Baltic Air Policing detachment and have been called into action in recent months.
Russia must confirm air strikes in Syria targeted IS militants and not opponents of the Assad regime, Foreign Secretary Philip Hammond has said.
Mr Hammond said actions in supports of the Assad regime were "incompatible" with the fight against IS militants.
"Russia has taken a major decision and assumed a heavy responsibility by this public propping up of Assad while he continues the terrorisation of his own people," Mr Hammond said in an address to the UN Security Council.
"It is not possible to be an effective part of the fight against ISIL and at the same time, with the same force, be attacking the moderate oppositionists resisting the oppression of the Assad regime.
"It's very important that Russia is able to confirm to the international community that the military action it has undertaken in Syria this morning is directed at ISIL and Al-Qaeda affiliated targets only."
US Secretary of State John Kerry said Washington would have "grave concerns" if Russia hit Syrian targets where IS fighters were not present.
Incursions by Russian jets into Turkish airspace have prompted a stern response from Nato's most powerful ally.
Here's what you need to know about this latest development in the increasingly bloody, complicated and volatile conflict.
Russia urges world leaders to unite to fight terrorism in Syria amid criticism that air strikes have targetted rebel fighters and civilians.