"We have seen enormous losses from Daesh (IS), more than 10,000 since the beginning of the campaign and this will end up having an impact," Blinken told French radio, without specifying whether the losses were in Iraq or Syria.
Blinken was speaking a day after an international conference in Paris in which 20 or so representatives of the anti-IS coalition pledged support for Baghdad's plan to claw back territory from the marauding jihadists who have conquered large parts of Iraq and Syria.
The coalition's strategy has been criticised for relying on air strikes without committing boots on the ground, but Blinken stressed there had been "significant progress".
Islamic State now controls "25 percent less of Iraq after nine months, a lot of their equipment has been destroyed and many Daesh members have been eliminated," Blinken said.
He nevertheless acknowledged the "resilience" of the group after the coalition has launched about 4,000 air strikes on them.
US officials have in the past put the militants' total estimated force at 30,000.
But a State Department spokeswoman, Marie Harf, cautioned that Islamic State continued to replace fighters "taken off the battlefield."
In a separate French radio interview, Iraq's ambassador to France, Fareed Yasseen, said the allies had heeded Baghdad's calls for more weapons to combat the group.
"The Americans have promised us and will shortly deliver missiles that will make the difference against these truck bombs ... which made us lose Ramadi," a key Iraqi city close to the capital.
"The French will be giving us similar weapons, ammunition and we are discussing other cooperation projects," the ambassador told Europe 1 radio.
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