A US State Department official has claimed that more than 10,000 Islamic State militants have been killed by its coalition forces since the bombing campaign began nine months ago.

However, this number is disputed by some experts, citing lack of hard evidence that can be checked by an independent evaluator.

US Deputy Secretary of State Tony Blinken cited the contentious figure in an interview on Tuesday with France Inter after meeting Iraqi Prime Minister Haider al-Abadi and other coalition members in Paris.

"Indeed, when you act against a force like Daesh, which is a terrorist force with a totalitarian ideology, and that does not fear death, we recorded an enormous loss for Daesh," Blinken said, referring to ISIS by its insulting Arabic name. "More than 10,000 since this campaign started. And this will eventually have an effect."

Blinken contested reports that actions taken by the US-led coalition forces were miserably failing to leave a dent on ISIS' forces.

"In fact, what we see, and what we saw today, is that there is an important progress but also the fact that Daesh remains extremely resilient and capable of taking initiatives," said Blinken, even if ISIS overran last month the strategic Iraqi city of Ramadi.

Blinken also said the Islamist extremist group controls 25 percent less territory than before, and coalition bombings have killed ISIS fighters and destroyed a lot of their military equipment.

"At the same time, we saw what happened in Ramadi and we take into consideration not only progress but also tactical defeats," he said.

Blinken's figure has been questioned by Jim Miklaszewski, Robert Windrem and Jon Schuppe of NBC News.

"The US military, which officially does not consider body counts as measures of success, hasn't explained how exactly it compiles ISIS casualties," Miklaszewski, Windrem and Schuppe said. "Analysts believe that the numbers come from various sources, including satellite imagery, reports from partners on the ground, and claims from ISIS itself."

NBC News maintained that body counts "are considered a poor method of showing progress."

"We are spending millions a day, so at some point you want to say what you've accomplished," said Patrick Skinner of security consulting firm Soufan Group.

More than 4,000 air strikes have been launched against ISIS.

Analysts said ISIS has the capability to adequately replace their killed comrades via different recruitment and conscription methods.

"US authorities have been wary of stating publicly how many ISIS militants are thought to have been killed since the campaign began," wrote Laura Smith-Spark and Noisette Martel of CNN. "US Ambassador to Iraq Stuart Jones told Al Arabiya television in January that an estimated 6,000 fighters had been killed."

The Central Intelligence Agency (CIA) placed the number of ISIS deaths at between 20,000 and 32,000 fighters.

Officials from the Department of Defence revealed that Blinken's estimate "of the number killed is correct, but was not intended for release."