The RAF is stepping up attacks on self-styled Islamic State forces in the city of Mosul as part of a bid to cripple the group's last major stronghold in Iraq, Michael Fallon says.
It comes as Iraqi government forces are "on the cusp" of retaking the city - held by IS since 2014, the UK Defence Secretary says.
Meanwhile, a senior military commander says IS has been firing surface-to-air missiles at coalition planes.
But so far no aircraft has been hit.
Lt-Gen Mark Carleton-Smith said the military had assumed IS possessed missiles capable of bringing down an aircraft but had not been targeted by them until recently.
Missiles fired at coalition aircraft have been detected as they were launched, enabling the targeted planes to take defensive action using flares, which cause the missiles to divert away from the plane.
'It will fall soon'
The Iraqi government and Kurdish Peshmerga forces intend to launch an offensive on IS in Mosul in the coming weeks.
Mr Fallon told reporters: "Having spoken to the commanders of the troops involved, their self-belief and determination is very clear.
"Though Mosul is a large and complex city, it will fall and will fall soon."
By Jonathan Beale, BBC defence correspondent
Over the past two years British warplanes have carried out more than a thousand airstrikes, in which they have dropped more than 2,000 bombs and missiles on what they say are IS targets in both Syria and Iraq.
They are not the only ones who've been looking for targets.
For the first time, the RAF has confirmed its aircraft have been fired at too - with IS anti-aircraft fire and shoulder-launched surface to air missiles - though they say they've always dealt with the threat.
Defence Secretary Michael Fallon said the focus of the air campaign was now turning towards Mosul, the last major city in Iraq still under IS control.
He said the offensive to liberate the city would begin within weeks but he admitted that pushing IS out of Syria would be much harder.
He added that the air force, which is targeting more than 100 locations in and around the city, was now operating "at the highest tempo in a single theatre for over 25 years".
The US-led coalition, which Britain is part of, has conducted almost 9,600 strikes against IS in Iraq, since 2014.
Lt-Gen Carleton-Smith, deputy chief of the defence staff, said coalition airstrikes had destroyed nearly $1bn (£770m) of cash held in stockpiles by IS.
Mr Fallon also said that Russian warplanes had "almost certainly" carried out an attack on a UN convoy which killed at least 20 people on Monday.
He said: "Instead of inventing more and more unbelievable excuses it is time Russia came clean.
"If it was a mistake Russia should apologise."