US President Barack Obama has vetoed a bill allowing 9/11 families to sue Saudi Arabia, risking fierce public backlash and a rare congressional rebuke.

Key points: Legislation allowing 9/11 victims to sue Saudi Arabia for compensation passed earlier in month by Congress

Legislation allowing 9/11 victims to sue Saudi Arabia for compensation passed earlier in month by Congress Most hijackers were Saudi citizens, but no link with Saudi Government has been proven

Most hijackers were Saudi citizens, but no link with Saudi Government has been proven White House insists Mr Obama vetoed bill due to worries of setting "dangerous legal precedent"

Expressing "deep sympathy" for the families of the victims, Mr Obama said the law would be "detrimental to US national interests".

The White House tried and failed to have the legislation — allowing 9/11 families to launch civil suits, which was unanimously backed by Congress — from being substantially revised.

Mr Obama now faces the prospect of Republican and Democratic politicians joining forces to override his veto for the first time in his presidency.

Such a rebuke would overshadow his last months in office and show the White House to be almost fatally weak.

New York Senator Chuck Schumer, a Democrat who co-sponsored the bill, said he would press ahead with the override.

"This is a disappointing decision that will be swiftly and soundly overturned in Congress," Mr Schumer said.

"If the Saudis did nothing wrong, they should not fear this legislation.

"If they were culpable in 9/11, they should be held accountable."

Families of 9/11 victims have campaigned for the law, convinced that the Saudi Government had a hand in the attacks that killed almost 3,000 people.

Fifteen of the 19 hijackers were Saudi citizens, and declassified documents showed US intelligence had multiple suspicions about links between the Saudi Government and the attackers.

No link to the Saudi Government has been proven, and the Saudi Government denies any links to the plotters.

Terry Strada, whose husband Tom was killed in World Trade Centre Tower One, told AFP families were "outraged and very disappointed" by Mr Obama's decision.

She vowed that the group would now lobby congress to overturn the decision.

Republican presidential nominee Donald Trump described the decision as "shameful".

"That President Obama would deny the parents, spouses and children of those we lost on that horrific day the chance to close this painful chapter in their lives is a disgrace," he said.

"If elected president, I would sign such legislation should it reach my desk."

Mr Trump's Democratic opponent Hillary Clinton was in agreement.

Her campaign spokesman, Jesse Lehrich, said she would also sign the bill.

Behind the scenes, Riyadh has been lobbying furiously for the bill to be scrapped.

A senior Saudi Prince reportedly threatened to pull billions of dollars out of US assets if it becomes law, but Saudi officials now distance themselves from that claim.

AFP/Reuters