U.S. Deputy Secretary of State Tony Blinken has claimed that the U.S. coalition has killed more than 10,000 ISIS militants since the bombing campaign began nine months ago. However, some have claimed that number is difficult to independently verify without hard evidence.

According to Laura Smith-Spark and Noisette Martel of CNN, Blinken made those comments during an interview on Tuesday with France Inter after meeting in Paris with Iraqi Prime Minister Haider al-Abadi and other coalition members. He also claimed that Iraqi forces will be able to stand up to ISIS with coalition support.

"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."

According to CNN, the CIA estimated that there were between 20,000 and 32,000 ISIS fighters in total. U.S. intelligence officials indicated that ISIS had the capability to "adequately replace" slain fighters through various recruitment and conscription methods.

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

Blinken denied reports that coalition efforts against ISIS were failing miserably. CNN reported that ISIS managed to overrun the strategic Iraqi city of Ramadi last month.

"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," Blinken said.

Blinken added that ISIS 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," Blinken said.

However, Jim Miklaszewski, Robert Windrem and Jon Schuppe of NBC News questioned whether the U.S. claim of 10,000 dead ISIS militants was believable. Over 4,000 strikes have been launched against the Islamic terror group.

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

NBC News contended that body counts "are considered a poor method of showing progress," adding that such reporting tactics were used ineffectively during the Vietnam War.

"The U.S. 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 wrote. "Analysts believe that the numbers come from various sources, including satellite imagery, reports from partners on the ground, and claims from ISIS itself."

Officials from the Department of Defense admitted to NBC News that Blinken's estimate "of the number killed is correct, but was not intended for release." However, NBC News reported that "the number is accurate only in the context of the much broader operations carried out by other ISIS opponents."

"That includes the Kurds, Shiite militias being armed and advised by Iran, Iraqi forces and Syrian forces," Miklaszewski, Windrem and Schuppe wrote.

Laith Alkhouri, who works for security consulting firm and NBC News partner Flashpoint Intelligence, thought that Blinken's number was unbelievable. He noted that the number doesn't account for recent military victories by ISIS or is backed up with evidence from the U.S. government.

"The reality on the ground is that ISIS is capturing territory, not losing ground," Alkhouri said.