Mr Fallon told the BBC’s Andrew Marr show 1,000 troops would be sent next year and 3,000 in 2017, chiefly in response to the threat posed by Russia.
He accused Vladimir Putin of “sabre-rattling” and said it was important for the UK to commit to Nato’s mission.
Tory backbenchers and US officials have expressed concerns at UK defence cuts.
On President Putin, Mr Fallon told the BBC: “He’s clearly building up his conventional forces, he’s modernising his nuclear weapons.
“So it is sabre rattling and that’s why we have to continue to strengthen Nato, offer Nato more reassurance through these larger scale exercises.
“We’re doing our bit, but it’s very important we send Putin this message that we are determined in our commitment to the collective defence of Nato.”