The United States' military kept distance from an official statement made earlier this week, in which U.S. Air Force Major General Peter Gersten had said that the number of foreign fighters joining the Islamic State group had fallen by 90 percent. While acknowledging that the flow of foreigners joining ISIS in Iraq and Syria has weakened, some officials reportedly dismissed the figure. 

While Gersten, deputy commander for operations and intelligence in the U.S.-led anti-ISIS coalition, had said the group was able to recruit only about 200 foreign fighters in a month as opposed to 2,000 earlier, another spokesperson for the coalition said the official estimate of foreigners joining the terror group was higher.

"We believe the foreign fighter flow was 2,000 at one point and is now down to a quarter or less of that. That would equal roughly 500 fighters per month, or a drop of about 75 percent from the peak," U.S. Army Colonel Steve Warren told Reuters

U.S. Defense Secretary Ash Carter on Thursday reportedly said it was difficult to be "precise about these numbers."

Coalition officials have said in recent weeks that the Islamic State group has been financially weakened after some of its cash storage sites were destroyed, while some of its territory was retaken. 

"What we are now seeing is that they are running short of cash and they are looking for more imaginative ways to do things," U.K.'s Air Vice-Marshal Edward Stringer said on Thursday, according to the Guardian

"It is early days, but only in the last week we have heard from the sources that we have that the taxation system is becoming more arbitrary, more looking to fines, and so becoming less progressive and less easy to sell to the population."

Gersten had said that about 20 coalition strikes had blown up nearly $800 million worth of cash of the group, according to AFP.