President Barack Obama vetoed a bill that would allow the families of 9/11 victims to sue the government of Saudi Arabia.

Obama objected to the 'Justice Against Sponsors of Terrorism Act' on Friday, warning that if US citizens are allowed to take the Saudis into court, then foreign countries could do the same to the United States.

Congress will try to override the president's veto as early as Tuesday. If the bill receives the two-thirds vote necessary in both the House and Senate, it will be the first of Obama's presidency.

Many families of 9/11 victims have campaigned relentlessly 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, but no link to the government has been proven. The Saudi government denies any links to the plotters.

President Barack Obama has vetoed a bill that would allow the families of 9/11 victims to sue the government of Saudi Arabia

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

Congress is expected to move rapidly, since lawmakers are eager to return home to campaign ahead of the November election.

Senate Majority Leader Mitch McConnell's office said the Senate would take up the override 'as soon as practicable in this work period.'

The bill would end the immunity foreign countries have from lawsuits in the US, and allow federal civil suits to be brought against countries if they are deemed responsible for a US terror attack.

But the president warned that the bill would be 'detrimental to US national interests' by chipping away at the concept of sovereign immunity.

Doing away with it could mean the same courtesy would no longer be extended to the US - opening up Americans diplomats and servicemen to lawsuits in foreign countries.

The administration was also apprehensive about undermining a longstanding yet difficult relationship with Saudi Arabia.

The Saudis remain a key ally in combating the spread of terrorism throughout the Middle East, and serve as a counterweight against Iran's influence in the region.

Just two days before, the Senate gave the green light to a $1.15 billion sale of military equipment to Saudi Arabia.

Some politicians backed the sale, saying the US needed to support its ally to maintain stability in the region, combat terror groups like Isis and al-Qaeda, and keep a check on neighboring Iran.

In a statement accompanying his veto message, Obama said he had 'deep sympathy' for the 9/11 families and their desire to see justice for their relatives.

But he also made the case for financial repercussions, writing: 'If any of these litigants were to win judgments — based on foreign domestic laws as applied by foreign courts — they would begin to look to the assets of the US Government held abroad to satisfy those judgments, with potentially serious financial consequences for the United States.'

THE POSSIBILITY OF OFFICIAL SAUDI INVOLVEMENT IN 9/11 The possibility of official Saudi involvement in 9/11 has hung over its diplomatic relationship with the US, since fifteen of the 19 men who carried out the attacks were Saudi nationals. In July, 28 declassified pages of a congressional report into were released, stating two of the hijackers were 'in contact with, and received support or assistance from individuals who may be connected to the Saudi government'. In August, CNN uncovered evidence of a link between the alleged al Qaeda operative and a company associated with a key member of the Saudi royal family, the former Saudi Ambassador to the United States Prince Bandar bin Sultan. While the alleged association with Bandar does not provide direct evidence the prince was complicit in the 9/11 attacks, it does raise questions about Saudi Arabia's involvement. In a transcript released last week, accused al-Qaeda bomb-maker Ghassan Al-Sharbi told military officials at Guantanamo Bay that he believed an unnamed member of the Saudi royal family was part of an effort to recruit him for violent extremist acts before the September 11 attacks. Al-Sharbi is believed to have learned how to fly with the 9/11 hijackers but did not take part in the attacks. In April, it also emerged that his flight certificate was found in an envelope from the Saudi embassy in Washington. Shortly before his arrest in 2002, Al-Sharbi buried a bundle of documents, which is believed to have included the certificate. The cache was discovered by US authorities and details, written in a memo known as Document 17 in 2003 and released without fanfare by investigators last year. They were brought to the public's attention when an activist discovered them and wrote about them on his website in April of this year. According to the Times, activist Brian McGlinchey said: 'The envelope points to the fundamental question hanging over us today: to what extent was the 9/11 plot facilitated by individuals at the highest levels of the Saudi government?' Advertisement

The possibility of official Saudi involvement in 9/11 has hung over its diplomatic relationship with the US, as fifteen of the 19 men who carried out the attacks were Saudi nationals. Pictured, the Foreign Minister of Saudi Arabia Adel Al Jubeir

A senior Saudi Prince reportedly threatened to pull billions of dollars out of US assets if the bill became law, but Saudi Foreign Minister Adel bin Ahmed Al-Jubeir denied the reports in May.

He did say, however, that investor confidence in the US would shrink if the bill was enacted.

Senator Chuck Schumer of New York, one of the most prominent Democrats urging Obama to sign the bill, rallied behind an override and predicted Obama's veto would be 'swiftly and soundly overturned in Congress.'

'The families of the victims of 9/11 deserve their day in court, and justice for those families shouldn't be thrown overboard because of diplomatic concerns,' Schumer said.

Families of the victims spent years lobbying lawmakers for the right to sue the kingdom in US court for any role elements of Saudi Arabia's government may have had in the attacks.

Kristen Breitweiser, whose husband died in 9/11, told the New York Times: 'It is reprehensible that one man is standing between justice for the murder of 3,000 people and this legislation becoming law.

'The president and the Congress should be listening to American citizens, not a bunch of lobbyists who represent a foreign nation,' she said, in a reference to the powerful figures the Saudi government has turned to for support.

Both presidential candidates Hillary Clinton and Donald Trump said they would have signed the bill if they were in office.

The possibility of official Saudi involvement in 9/11 has hung over its diplomatic relationship with the US, since fifteen of the 19 men who carried out the attacks were Saudi nationals.

In July, 28 declassified pages of the 9/11 report were released, stating two of the hijackers were 'in contact with, and received support or assistance from individuals who may be connected to the Saudi government'.

Declassified documents revealed a link between an alleged al Qaeda operative and a company associated with a member of the kingdom's royal family, Prince Bandar bin Sultan, pictured

In August, CNN uncovered evidence of a link between the alleged al Qaeda operative and a company associated with a key member of the Saudi royal family, the former Saudi Ambassador to the United States Prince Bandar bin Sultan.

While the alleged association with Bandar does not provide direct evidence the prince was complicit in the 9/11 attacks, it does raise questions about Saudi Arabia's involvement.

In a transcript released last week, accused al-Qaeda bomb-maker Ghassan Al-Sharbi told military officials at Guantanamo Bay that he believed an unnamed member of the Saudi royal family was part of an effort to recruit him for violent extremist acts before the September 11 attacks.

Al-Sharbi is believed to have learned how to fly with the 9/11 hijackers but did not take part in the attacks.

In April, it emerged that his flight certificate was found in an envelope from the Saudi embassy in Washington when they arrested him in 2002.

Shortly before his arrest, he buried a bundle of documents, which is believed to have included the certificate.

The cache was discovered by US authorities and details, written in a memo known as Document 17 in 2003 and released without fanfare by investigators last year.

They were brought to the public's attention when an activist discovered them and wrote about them on his website in April of this year.

According to the Times, activist Brian McGlinchey said: 'The envelope points to the fundamental question hanging over us today: to what extent was the 9/11 plot facilitated by individuals at the highest levels of the Saudi government?'

Prince Bandar (right) was a former Saudi Ambassador to the US known to have a very close relationship with George W. Bush. Pictured, the pair at Bush's Crawford ranch in Texas

Proponents of the bill, including the support group 9/11 Families United For Justice Against Terrorism, dismissed fears that the US could become the target of reciprocal lawsuits.

One sponsor, Republican Peter King, of New York, argued foreign governments cannot look the other way if terrorist activities are being plotted or launched from inside their borders.

In the run-up to Obama's veto, the White House said the system the US uses to identify and punish countries that support terrorism was set by law and is more effective than a 'patchwork' of legal decisions.

The Senate passed the bill in May, and the House voted on September 9, two months after Congress released the 28 declassified pages from a congressional report into 9/11.

Brian McGlinchey, director of advocacy for the website 28pages.org, said making the documents public 'strengthened the resolve of 9/11 families and other advocates of justice to bring about the enactment' of the bill.