The US Hellfire missile that killed al-Qaida leader Ayman al-Zawahri

"One of their utilities is in opening up vehicles and other obstructions to get to the target without having to use an explosive warhead," he said.

US officials and experts made clear this week that avoiding civilian casualties was a crucial element in the choice of weapon.

Less than a year ago, a US drone strike - using a more conventional Hellfire missile - struck a white Toyota Corolla sedan in a Kabul neighborhood and killed 10 civilians around and near the car, including seven children.

In the midst of the chaotic US military withdrawal from Afghanistan, American forces believed there were explosives in the car and that it posed an imminent threat to troops on the ground. It was, military leaders said, a "tragic mistake"

One former US official said the likely choice of an R9X is an example of the administration's effort to find ways to minimise collateral damage and prevent the loss of innocent life. That missile is a very accurate weapon that strikes in a very small area, said the official, who spoke on condition of anonymity to discuss counterterrorism operations.

An administration official said Monday that the US investigated the construction of the house where al-Zawahri was staying in order to ensure that the operation could be done without threatening the structural integrity of the building and also minimising the risks of killing civilians, including members of his family who were in other parts of the house.

The choice of a missile is ultimately one part of reducing the possibility of killing civilians or causing other collateral damage.

"I would say this is by far a lower-risk option," said Tom Karako, an expert on missile defense at the Washington-based Center for Strategic and International Studies. Using the Hellfire, he said, "reflects a high degree of caution as opposed to a riskiness."

Is the US providing Ukraine with drones that can fire Hellfire missiles?

No. While the US has delivered billions of dollars in military assistance to help Ukraine fight the invading Russian troops, it is wary of providing weapons that could fire deep into Russia, potentially escalating the conflict or drawing the US into the war.

As a result, the US so far has not provided Hellfire missiles or drones that could fire them. Instead, the US has delivered smaller, so-called kamikaze drones, such as the Switchblade and Phoenix Ghost, which instead of firing missiles, explode when they hit a target.