Over 15,000 Islamic State members have been killed by US-led coalition airstrikes since the campaign began nearly a year ago, as the militant Islamist group displays a dichotomy of weakened resilience.

The US-led coalition conducted more than 30 airstrikes on the Islamic State within 24 hours, destroying more than five IS vehicles, six tactical units and a tunnel near Mosul, Iraq, that the militant group used to move members to fight against opposing Iraqi military and Kurdish Peshmerga forces, officials announced Wednesday.

More than 5,500 airstrikes have been carried out by the US-led coalition against the Islamic State in Syria and Iraq since Aug. 8, 2014. The CIA estimated the militant group's membership last year to be in a range between 20,000 and 31,500

The membership estimates remain about the same this year, according to USA Today -- meaning the Islamic State has been able to successfully replace its fallen fighters.

Due to the airstrikes, the Islamic State has been forced to disperse into smaller groups, making them less likely to seize large territories. The US military has previously warned that casualty numbers do not accurately measure progress in the conflict against the militant group.

Former Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey said in Iraq earlier this month that combating the Islamic State militarily and ideologically will last for years.