London: Around 100 British military personnel will be sent to Estonia, Latvia and Lithuania in the Baltic region, the Ministry of Defence said on Thursday.
A further 25 will be involved in a continuing training mission in Ukraine, the MoD said.
Defence secretary Michael Fallon said troops in the Baltic region would deter Russian aggression beyond Ukraine and reassure eastern European NATO members.
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.
He said British troops will be involved in training, evaluation and capacity-building missions in Poland and the three Baltic states.
"Russia is making a very serious situation in Syria much more dangerous. We will be calling on Russia specifically to stop propping up the Assad regime, to use their influence constructively to stop Assad bombing his own civilians and themselves to avoid the use of unguided munitions in areas that are not being controlled by ISIL," he said.
"If Russia wants to help here, the single most helpful thing they can do is use their influence on Assad to stop barrel-bombing his own civilians, their children, his own cities and villages. That's how Russia could help to resolve this conflict," he added.
In June, Royal Air Force (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 British teams in Ukraine have trained nearly 1,600 members of the Ukraine Armed Forces at eight training sites.
The operation is on course to have trained more than 2,000 Ukrainian troops by the end of the financial year.