The additional 250 troops will bring Britain’s contingent of advisers to more than 500, he said, as allied ministers met in Washington to plan moves to defeat the jihadists.
“We will be sending an additional 250 troops into the theater over the next few weeks,” Fallon said.
“Other countries are being asked to look for their contribution to see what more they can do.”
Earlier, US Defense Secretary Ash Carter had said the US-led coalition fighting the Islamic State group was drawing up a plan to liberate its remaining bastions.
Allied air power and military trainers are helping Iraqi government, Kurdish militia and Syrian anti-IS fighters push the jihadists back to Raqa, Syria and Mosul, Iraq.
There, in the coming months, commanders hope to defeat the IS “caliphate” — its heartland territory — but they admit its broader network will be harder to crack.