Bombings, desertions and ground offensives have hit the Islamic State (Isis) where it hurts as the terror group has seen its fighting ranks drop by up to 12,000 members, according to US. The White House cited new intelligence estimates as putting the number of IS (Daesh) militants in Iraq and Syria between 19,000 and 25,000 down from 20,000-31,000 in 2014.

"Now, that means they continue to be a substantial threat, but the potential numbers have declined," said White House spokesman Josh Earnest. He explained the sharp decrease in numbers was down to a series of factors, including coalition air strikes and attacks from US allies on the ground.

Iraqi forces and Kurdish militants have been making gains in Iraq and Syria in recent months, delivering symbolically important victories in Ramadi and Sinjar.

"As a result of those efforts, IS has sustained significant casualties," Earnest said, adding that the drop also underscored successful international efforts to cut recruitment lines and stem the flow of foreign fighters. "IS is having more difficulties than they've had before in replenishing their ranks," Earnest said.

Deserters also played a role in trimming IS military manpower, according to the National Security Council. "The decrease reflects the combined effects of battlefield deaths, desertions, internal disciplinary actions, recruiting shortfalls, and difficulties that foreign fighters face travelling to Syria," National Security Council spokeswoman Emily Horne told Reuters.