A US judge has told the families of those killed in the 9/11 terror attacks they cannot sue Saudi Arabia over claims the country helped Al Qaeda carry out the atrocity.

Manhattan District Judge George Daniels said that because Saudi Arabia enjoys sovereign immunity, which means a state cannot commit a legal wrong and therefore can't be prosecuted in either criminal or civil cases, any attempt to sue the kingdom would be "futile".

He said that, despite the best efforts of families to provide additional evidence against the country, their claims would not “strip defendants of sovereign immunity.”

9/11 attackers countries of origin 15: Saudi Arabia

2: United Arab Emirates

1: Egypt

​1: Lebanon

"The allegations in the complaint alone do not provide this court with a basis to assert jurisdiction over defendants," Daniels wrote on Tuesday.

Families, as well as insurers that covered losses from the buildings and local businesses, had secured testimony from Zacarias Moussaoui, a former al-Qaeda operative imprisoned for his actions.

But this effort proved fruitless as Judge Daniels refused to consider the case in Manhattan.

Lawyers from both parties declined to on comment on the situation.

9/11 was carried out by 19 men, 15 of whom were from Saudi Arabia, and killed 2,996 people. The Pentagon and the two World Trade Centre towers were targeted in an attack that horrified the world.

In the years following the tragedy, civil action against Saudi Arabia has proceeded in fits and starts.

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Trial judges – including Judge Daniels – had ruled the country was entitled to immunity twice before under the federal Foreign Sovereign Immunities Act. In 2013, claimants appeared to make headway when the 2nd US Circuit Court of Appeals in New York revived the lawsuit following a decision in 2011 that allowed a similar suit against Afghanistan.