Barack Obama has met with the king of Saudi Arabia in Riyadh to discuss counter-terrorism and the threat of ISIS, just as fresh information emerged that appears to connect the Arab nation and the September 11 attacks.

President Obama's uncomfortable meeting came after officials revealed that the flight certificate of Al-Qaeda bomb maker Ghassan Al-Sharbi was discovered hidden in an envelope from the Saudi embassy in Washington when they arrested him in 2002.

On top of that, there is mounting pressure on Mr Obama at home to declassify a 28-page section of a Congressional report which many believe will point to Saudi involvement in the 2001 plane hijackings, which killed nearly 3,000 people and launched the War on Terror.

The claims are highly significant as President Obama arrived in Saudi Arabia this morning so he could meet with officials. He is pictured with King Salman, above

President Obama will be spending the next 24 hours in the country. This is his fourth visit while in office

Al-Sharbi, who became an Al Qaeda bombmaker, is believed to have learned how to fly with the 9/11 hijackers but did not take part in the attacks. 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, were released without fanfare by investigators last year. They were only brought to the public's attention when an activist discovered them and wrote about them on his website earlier this week.

Activist Brian McGlinchey claimed the new details of the flight certificate would lead to people questioning the extent government individuals were involved, according toThe Times.

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? Activist Brian McGlinchey

He 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?'

Victims' families have been pushing Congress for the right to sue Saudi Arabia over the death of their loved ones as well as calling for the declassification of the report.

Previous court decisions have ruled that there is insufficient evidence to find Saudi Arabia culpable in the terror attacks, which is why they are now calling for the release of the 28 pages from the 9/11 congressional report which is believed to show a stronger connection to Saudi funding of the attacks.

This has put Obama in a difficult position, with 9/11 families accusing him of siding with the Kingdom and Saudi officials threatening to sell hundreds of billions of dollars of American assets if Congress passes a bill that would allow the government to be sued over the attacks.

The talks between the leaders (above) are expected to address the Saudi-led military campaign against Shiite rebels and their allies in neighboring Yemen

Obama, accompanied by the Commander of 89th Airlift Wing Colonel John Millard, walks to board Air Force One on the way to Saudi

Saudi Foreign Minister Adel al-Jubeir has said his country would sell up to $750 billion in US treasury securities and other assets before the bill puts them in jeopardy.

The administration has tried to stop Congress from passing the legislation, a bipartisan Senate bill. Earlier this week, Josh Earnest, the White House press secretary, indicated that President Obama would veto any such legislation.

Paul Ryan, the House speaker, has also refused to back the legislation, saying that lawmakers need to review the bill 'to make sure that we're not making mistakes with our allies'.

Fifteen of the nineteen men who hijacked four planes and flew them into targets including the World Trade Center in 2001, were Saudi citizens

The increasingly strained relationship between the two countries was laid bare in Riyadh this morning, when Mr Obama arrived at King Khalid International Airport, to be greeted by Prince Faisal bin Bandar bin Abdulaziz Al Saud, the governor of Riyadh, as opposed to the king himself.

Saudi state television did not immediately air Obama's arrival, which was unusual since right before Air Force One landed, State TV showed King Salman greeting other senior officials from Gulf nations arriving for the Gulf Cooperation Council summit.

Mustafa Alani, a security analyst at the Gulf Research Center, said the decision to send a lower ranking official to greet Mr Obama was intended to send a clear message that they have little faith in him.

'He will find a leadership that's not ready to believe him,' he said. 'The Saudis had disagreements with previous presidents. Here you have deep distrust that the president won't deliver anything.'

Obama eventually did meet King Salman at Riyadh's Erga Palace before the six-nation GCC summit opens on Thursday.

'The American people send their greetings and we are very grateful for your hospitality, not just for this meeting but for hosting the GCC-U.S. summit that's taking place tomorrow,' Obama said, referring to the six-nation Gulf Cooperation Council summit.

King Salman offered similarly gracious words through a translator, saying: 'The feeling is mutual between us and the American people.'

The White House later said that the pair 'really cleared the air.'

The relationship between the US and Saudi Arabia is not helped by the Middle Eastern nation's concerns that the US is shifting its attention to improving relations with Iran.

When Obama called on Saudi Arabia to 'share' the Middle East with Iran and stop the proxy wars across the region, it infuriated the kingdom.

'I don't think any US president has ever been so outspoken about Saudi Arabia,' Toby Matthiesen, a Middle East specialist at St Antony's College, University of Oxford, told the news agency AFP. He added that it was 'embarrassing' for the Saudis.

Meanwhile Mr Obama and Ash Carter, his Defense Secretary, used the trip to appeal to other Gulf nations to do more to support Iraq economically and politically. Mr Carter asked them to help with the reconstruction of the cities of Ramadi and Hit as well as Anbar province, which have been left devastated by ISIS.

Fifteen of the nineteen men who hijacked four planes and flew them into targets in New York and Washington in 2001 were Saudi citizens, though the country has always denied having any role in the attacks.

A U.S. commission established in the aftermath of the attacks also concluded there was no evidence of official Saudi connivance. However, the White House has been under pressure to declassify a 28-page section of the report that was never published on the grounds of national security.

President Obama will decide whether to declassify the sealed documents by June. During an interview with Charlie Rose this week, he said that James Clapper, the director of National Intelligence, was nearly done with a review of the documents to ensure that whatever is made public does not damage US national security interests.

'I have a sense of what's in there', Mr Obama told Rose.

Donald Trump, the Republican presidential frontrunner, believes the pages should be released.

'I think I know what it's going to say,' Mr Trump said on Fox & Friends. 'It's going to be very, very profound, having to do with Saudi Arabia and Saudi Arabia's role on the World Trade Center, and the attack.'

Former US Senator Bob Graham earlier this month said Saudi officials are against the bill that would make it easier for families to sue.