A top US Air Force general insisted Friday the American-led air campaign against the Islamic State was effective, rejecting criticism that it was too slow or overly cautious.

The bombing raids against the IS jihadists in Iraq and Syria have had a "profound effect on the enemy" and taken out "more than a 1,000 enemy fighters a month from the battlefield," said Lieutenant General John Hesterman, head of the air fleet under US Central Command.

Coalition strikes have helped ground forces in Iraq and northern Syria regain territory from the IS and destroyed most of the group's oil refining capacity, Hesterman told reporters via telephone from Qatar.

President Barack Obama's administration has come under criticism at home and abroad over the air campaign, with some lawmakers and retired air force officers accusing Washington of imposing too many limits on military pilots.

Despite thousands of US-led coalition bombing raids since August, the IS jihadists have gained ground in Syria and last month captured the Iraqi city of Ramadi in a stunning defeat for the Baghdad government army.

Although air strikes failed to prevent the fall of Ramadi, Hesterman delivered a spirited defense of the coalition campaign, saying it could not be compared to previous air wars with more conventional targets.

The general acknowledged that aircraft in roughly 75 percent of all strike flights return without dropping bombs, but he said that was because the IS militants were not a traditional army and were moving among the local civilian population.

"Targeting a field army is relatively easy. That's not what we're doing," Hesterman said. "The comparisons being made to conflicts against field armies in nation-states don't apply in this case."

"This enemy wrapped itself around a friendly population before we even started," he said.

- Trusting pilots -

He said that coalition warplanes are in the air round the clock "to get after this enemy whenever we have the opportunity whenever they show themselves."

"Sometimes they don't and we bring those weapons back. That's not because we're seeing them and not killing them."

Some American pilots have complained to US media that they faced cumbersome rules in the skies over Iraq and Syria that hampered their ability to go after the IS jihadists.

Hesterman, however, said coalition pilots were not hamstrung and that the approval for most strikes was "measured in minutes, not hours or halves of hours."

"The thought that we don't trust our pilots is just wrong," he said.

Critics of the air campaign have demanded the deployment of forward air controllers with Iraqi or Kurdish forces to help guide air strikes against the IS extremists.

Hesterman said spotters would "probably" be helpful but were not necessary "so far."

The general, who oversees air forces operating over Iraq and Syria, said that the IS jihadists did not appear out in the open in large numbers in the battle for Ramadi.

"In Ramadi, you know, if the enemy (had) massed at Ramadi, they would be dead," he said.

He said ultimately local forces -- backed up by air strikes -- would have to seize back ground from the IS group.

"Air power doesn't hold and govern territory."

The coalition also had to take care to distinguish from the air between Iraqi government forces and the IS militants, while also taking pains to avoid inadvertently killing civilians, he said.

"It has never been more difficult to identify friend from foe than it is right now in Iraq. It is nearly impossible to tell them apart when they dress nearly the same and use the same equipment," the general said.

"So imagine if those strikes had been made, even a fraction of them, what we call blue-on-green fratricide. My opinion is the coalition would have unwound some time ago."

According to US Central Command, which oversees forces in the Middle East, there have been 15,675 coalition strike missions over Iraq and Syria, and bombs were dropped in 4,423 of those flights.