WASHINGTON — U.S. intelligence estimates the current strength of the Islamic State's force in Iraq and Syria is between 20,000 and 25,000 fighters. That compares to an estimated 19,000 to 31,000 fighters more than a year ago, as the U.S.-led bombing campaign against the militants got underway, a senior government official said Tuesday.

The official, who asked not to be named because he was not authorized to discuss the figures before they are formally released, cautioned that the numbers are a rough estimate.

Airstrikes also had an impact on the terror group’s strength. An estimated 28,000 Islamic State fighters have been killed by the bombings, according to the U.S.-led coalition. Hundreds more have been killed in ground fighting in Iraq and Syria.

The numbers suggest that the Islamic State, also known as ISIL or ISIS, has been able to replenish its ranks, despite the heavy losses.

U.S. defense officials said the fighting ability of the militants has declined, as the extremist group has had to rely more on forced conscription in territory it controls.

“Voluntary recruitment is no longer sufficient to meet their needs,” Col. Steve Warren, a coalition spokesman, said in a briefing Wednesday from Baghdad.

Islamic State militants dropped their weapons and fled the battlefield during recent clashes with Iraqi security forces. A year ago, by contrast, Iraqi forces faced committed militants willing to die for a cause.

“We have seen an uptick in reports of desertion, and more importantly, we have seen ISIL execute its own fighters for fleeing the battlefield,” Warren said.

The Islamic State has lost 40% of the Iraqi territory it controlled at its peak in Iraq last year, as government security forces backed by coalition airstrikes have retaken such cities as Ramadi, Tikrit and Sinjar, according to the coalition.