WASHINGTON (NYTIMES) - The blast that killed the Islamic State of Iraq and Syria (ISIS) group's leader during a Special Operations raid in north-western Syria last week was most likely caused by a large bomb the terrorist rigged to destroy most of his third-floor residence, senior United States military officials said Thursday (Feb 10).

The explosion was so powerful that military officials now suspect that a child found dead on the building's second floor was killed by the blast's concussive force, not in a firefight between the child's parents and the commandos.

The child had no visible injuries from gunshots or falling debris, the officials said.

The Pentagon has acknowledged seven deaths - four civilians and three ISIS fighters - in the raid to capture or kill the leader, Abu Ibrahim al-Hashimi al-Quraishi.

But the military officials acknowledged Thursday that more bodies might have been recovered from the rubble after the commandos had left the scene. Rescue workers have said women and children were among at least 13 killed during the assault.

New details about the pre-dawn assault are emerging a week after US President Joe Biden said he had ordered commandos to seize the ISIS leader, rather than bomb the entire three-story building, to minimise the risks to civilians.

Pentagon officials have said that 10 people, including eight children, were safely evacuated. Defence Secretary Lloyd Austin said the military would review whether the mission had harmed civilians.

The two-hour raid in the town of Atmeh near the Turkish border came days after the end of the largest US combat involvement with the ISIS group since the jihadis' so-called caliphate fell three years ago.

US forces backed a Kurdish-led militia in north-eastern Syria as it fought for more than a week to oust Islamic State fighters from a prison they had occupied in the city of Hassakeh.

The battle for the prison killed hundreds of people and offered a bleak reminder that even after the collapse of the caliphate, and now the death of al-Quraishi, the group's ability to sow chaotic violence persists.

Indeed, a United Nations counterterrorism report issued this week estimated that the ISIS group still retains 6,000 to 10,000 fighters across Iraq and Syria, "where it is forming cells and training operatives to launch attacks."

Also this week, the State Department offered a reward of up to US$10 million (S$13.45 million) each for information leading to the identification or location of Sanaullah Ghafari, leader of Islamic State Khorasan, or ISIS-K, the group's branch in Afghanistan.

The terrorist group claimed responsibility for an attack at Kabul's international airport on Aug 26 that killed 13 US service members and as many as 170 civilians during the US-led evacuation.

On Thursday, two senior US military officials described the planning and execution of the raid to a small group of reporters on a teleconference. They spoke on the condition of anonymity to discuss operational matters.

US officials have previously said al-Quraishi, also known as Hajji Abdullah, lived with his wife and two children on the building's third floor. He left the building only occasionally to bathe on the rooftop.

He relied on a top lieutenant who lived on the building's second floor and who, along with a network of couriers, carried out his orders to ISIS branches in Iraq, Syria and elsewhere in the world.

A Syrian family with no apparent connection to the terrorist group was living on the first floor.

Shortly after the commandos arrived just after midnight, warnings shouted in Arabic over bullhorns urged occupants on the first floor - as well as anyone else - to evacuate. A man, a woman and four children fled the first floor.

At almost the same time, a huge explosion - much bigger than a suicide vest with five to 10 pounds of explosives, officials said Thursday - ripped through the third floor. The blast was so powerful that bodies, including al-Quraishi's, were blown out the window.

Mr Biden said last week that al-Quraishi died when the terrorist exploded a bomb that killed him as well as members of his family.

Military officials said on Thursday that they had no proof that al-Quraishi detonated the bomb but thought so, given his position. The officials emphasised that the US commandos did not attack the third floor or detonate any explosives, and caused none of the casualties.

After the blast, commandos stormed the building and engaged in a firefight with al-Quraishi's lieutenant and his wife, who were barricaded on the second floor with their children. Both died, as did one child, but four children were safely evacuated, US officials said.

A 13-year-old boy who was among those evacuated from the first floor described his family's terror at being taken from their home in the middle of the night.

The commandos had thrown his father to the ground and kicked him before picking him up and searching his body for weapons, the boy said, giving only his first name, Muhammad, for fear of retribution.

"I felt like I had reached my death and that there was no escape," he told a reporter for The New York Times two days after the raid. "I figured when I saw them throw my father to the ground that they were going to kill him, to shoot him."