By Kylie MacLellan

LONDON (Reuters) - The U.S.-led coalition effort against Islamic State is killing the group's fighters more quickly than it can replace them, a senior British general said on Tuesday, with more than 45,000 killed by coalition air strikes up to August last year.

On Tuesday, U.S.-backed Iraqi forces continued their offensive in Mosul, where several thousand Islamic State (IS) militants, including many who traveled from Western countries to join up, are believed to be based.

"We are killing Daesh at a rate that they simply can't sustain," said Major General Rupert Jones, deputy commander for the Combined Joint Task Force coalition, using an Arabic acronym for Islamic State.

"The enemy cannot sustain the attrition that they are suffering and therefore they lose terrain, they lose battles."

The top American commander in Iraq said earlier this month he believed U.S.-backed forces would recapture Islamic State's two major strongholds - Raqqa in Syria and Mosul in Iraq - within the next six months.

Jones said that while the battle would not be over with the fall of Mosul or Raqqa, it would be the beginning of the end.

"The inevitability of their destruction just becomes really a matter of time," he said, adding that the group's leadership was now focused on little more than survival.

"BRUTAL REGIME"

The coalition estimates that the number of IS fighters in Iraq and Syria is at its lowest level in more than 2.5 years, with the group having lost 62 percent of the territory it once controlled in Iraq and 30 percent in Syria.

Jones said the number of foreign fighters traveling to join IS had dropped by between 75 and 90 percent, both due to it being harder to get in and out of Iraq and Syria and because the reality of doing so had been exposed as unappealing.

"The big idea that Daesh were putting out there, the kind of glamor ... has been exposed for what it is, it is a lie," he said. "They recognize that what you are actually signing yourself up to is going to live under a brutal regime."

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He said Islamic State had refocused its attention on radicalizing people outside Iraq and Syria to carry out attacks locally, so the coalition still had work to do on countering the group's ideology.

The coalition estimates that IS activity on Twitter has fallen by 45 percent since 2014, with 360,000 of the group's Twitter accounts suspended, and the lifespan of an IS-linked social media account reduced to less than two days.

(Reporting by Kylie MacLellan; Editing by Gareth Jones)