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.
Hundreds of Iranian troops reportedly arrived in Syria 10 days ago to take part in ground operations in rebel-held areas in the north, Reuters reports, citing Lebanese sources.
The offensive would accompany recent air strikes launched by Russia, the sources said.
Lebanon's ally Hezbollah is also preparing to join the operation, they added.
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.