The United States believes that Hamza bin Laden, a son of slain Al Qaeda leader Osama bin Laden and himself a notable figure in the militant group, is dead, a US official said on Wednesday.

The US official, who spoke on condition of anonymity, provided no further details, including when Hamza died or where.

President Donald Trump earlier on Wednesday declined to comment after NBC News first reported the US assessment. Asked if he had intelligence that bin Laden's son had been killed, Trump told reporters: "I don't want to comment on it."

Separately, the White House declined to comment on whether any announcement was imminent.

Hamza, believed to be about 30 years old, was at his father's side in Afghanistan before the September 11, 2001, attacks on the United States and spent time with him in Pakistan after the US-led invasion of Afghanistan pushed much of Al Qaeda's senior leadership there, according to the Brookings Institution.

Osama bin Laden was killed by US special forces who raided his compound in Pakistan in 2011. Hamza was thought to be under house arrest in Iran at the time, and documents recovered from the compound indicated that aides had been trying to reunite him with his father.

The New York Times reported that the United States had a role in the operation that led to Hamza's death, which it said took place in the past two years. Reuters could not immediately verify those details.

Still, the US government's conclusion appears to be a recent one. In February, the State Department said it was offering a reward of up to $1 million for information leading "to the identification or location in any country" of Hamza, calling him a key Al Qaeda leader.

Introduced by Al Qaeda's chief Ayman al-Zawahiri in an audio message in 2015, Hamza provided a younger voice for the group whose ageing leaders have struggled to inspire militants around the world galvanised by Islamic State, according to analysts.

Hamza called for acts of terrorism in Western capitals and threatened to take revenge against the United States for his father's killing, the US State Department said in 2017 when it designated him as a global terrorist.

He also threatened to target Americans abroad and urged tribal groups in Saudi Arabia to unite with Yemen's Al Qaeda in the Arabian Peninsula to fight against Saudi Arabia, it said.

In March, Saudi Arabia announced it had stripped Hamza bin Laden of his citizenship, saying the decision was made by a royal order in November 2018.