Washington (AFP) - Before blowing up a jihadist cash hoard in Iraq, the US military warned bystanders of an impending strike by using a Hellfire missile to deliver the wartime equivalent of a doorknock, an official said Tuesday.

It was the first time the Pentagon has conducted a "knock operation" in Iraq and Syria, and the inspiration came from watching the Israel Defense Forces (IDF) pioneer the controversial tactic in Gaza, Major General Peter Gersten said.

The Baghdad-based commander told Pentagon reporters that ahead of the strike on a cash-storage facility on April 5 in Mosul, the military learned that a woman, children and other "non-combatants" also were using the building.

He said the United States aims to avoid civilian casualties, and in this instance decided to warn occupants by exploding a missile just above the roof.

"We went as far as actually to put a Hellfire on top of the building and air burst it so it wouldn't destroy the building, simply knock on the roof to ensure that she and the children were out of the building," he said.

"Then we proceeded with our operations."

Ultimately, the woman died anyway because she ran back just after US forces launched bombs to blow it up.

"Much as we tried to do exactly what we wanted to do and minimize civilian casualties, post-weapons release, she actually ran back into the building," Gersten said. "That's ... very difficult for us to watch."

Gersten said several men had also fled the building. He did not say if they were IS jihadists.

"The men that were in that building, multiple men, literally trampled over her to get out," he said.

The coalition has carried out about 20 strikes on IS cash, blowing up as much as $800 million worth of cash in the process, Gersten said.

Critics of the 20-month-old US-led coalition attacking the IS group in Iraq and Syria say the military is overly cautious in avoiding civilian casualties.

In a move ridiculed by hawkish opponents in the US Congress and privately by some coalition partners, pilots dropped pamphlets before bombing trucks ferrying illicit oil around Syria for the IS group.

The IDF has for years warned occupants of buildings suspected of housing Hamas weapons to get out by "roof knocking."

The technique has drawn sharp criticism. Observers say occupants are sometimes killed in the warning strike, or even run up to their rooftops to see what happened -- only to be killed in the follow-up strike.