Michael Fallon: UK to send troops to Baltic region
About 100 British military personnel will be sent to Estonia, Latvia and Lithuania, the MoD has confirmed.
A further 25 will be involved in a continuing training mission in Ukraine.
Defence Secretary Michael Fallon said troops in the Baltic region would deter Russian aggression beyond Ukraine and reassure eastern European Nato members.
Mr Fallon, who is in Brussels for a Nato meeting, also called on Russia to change its strategy in Syria where it has been carrying out air strikes.
Russia's growing military involvement in the Syria conflict is expected to be high on the agenda of the Nato meeting.
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Nato states have expressed concern over Russia's backing for separatist rebels in eastern Ukraine and its bombing campaign in Syria.
In June, RAF Typhoons, which have been deployed to the Baltic region each year since May 2014, were scrambled from Estonia to intercept and shadow two Russian military aircraft over the Baltic Sea.
Nineteen UK teams in Ukraine have trained nearly 1,600 members of the Ukraine Armed Forces (UAF) at eight training sites.
The operation is on course to have trained more than 2,000 UAF troops by the end of the financial year.
By Jonathan Beale, BBC defence correspondent
Britain has already been sending troops to take part in exercises in the Baltics. The RAF has been sending Typhoon jets to the region to help patrol the skies.
It's all part of reassuring nervous Nato allies worried about their increasingly belligerent neighbour, Russia.
That's how this latest UK deployment should also be viewed.
About 100 British troops will be sent to the Baltics to take part in a Nato training mission that's already up and running.
It will be a "persistent" rather than a permanent presence.
An MoD source said he expected the British troops to be there 90% of the time.
Nato does not want to be accused of breaking previous agreements with Russia about building new military bases in eastern Europe.
This move is likely to irritate Moscow. But such a modest deployment won't cause alarm.
Mr Fallon said the UK deployment was "further reassurance for our allies... for Nato, for the Baltic states and for Poland."
He said the troops were part of a "more persistent presence by Nato forces" to respond to "any further Russian provocation and aggression".
The move forms part of the US-German Transatlantic Capability Enhancement and Training initiative, which co-ordinates military training and exercises in the Baltic States and Poland.
'Much more dangerous'
Later, Mr Fallon is expected to say: "We are committed to supporting the sovereignty of the democratic nations of Eastern Europe.
"We are already deploying RAF jets to the Baltics and providing crucial training to the Ukrainian armed forces.
"Now we will have a more regular drumbeat of troops deploying in the Baltics and Poland."
Sir Andrew Wood, former British ambassador to Russia, said Russian president Vladimir Putin would see the stationing of British troops in the Baltic states "as a provocation".
On Russia's intervention in the Syrian civil war, Mr Fallon said Russia was "making a very serious situation in Syria much more dangerous".
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 "moderate" rebel groups have been targeted.
Mr Fallon said: "We'll be calling on Russia specifically to stop propping up the Assad regime, to use their influence constructively to stop Assad bombing his own civilians."
Sir John Sawers, former head of Britain's MI6 foreign intelligence agency, said Russia's intervention in Syria was "a major step up" in its level of support for Assad's regime.
He told BBC Radio 4's Today programme the "reticence" of the West, in the wake of Iraq and Afghanistan, to deploy military power "had left a space into which Putin has inserted himself".