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 gave the United States one hour to clear the skies above Syria before starting its own air strikes on Syria, officials have told Reuters.
The officials said the US carried out its own strikes against IS targets near Aleppo, despite Russian demands to keep the skies clear.
One official said Russia's actions were dangerous because there had not been "deconfliction" talks, discussions whereby countries operating in the same military theater seek to avoid accidentally coming into conflict.
Meanwhile, the Russian Foreign Ministry said claims that the country's strikes were not aimed at IS targets were based on false information.
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.