Saudi Arabia has expressed “great concern” and warned of “serious unintended consequences”, following the US Congress’s decision to overrule President Obama and push through a law allowing legal action against the kingdom over the September 11 attacks.

Some analysts have speculated that Riyadh could retaliate by curbing trade with the US or restricting cooperation on security, a crucial relationship for US counter-terrorism and for peace efforts in Arab conflicts.

The Saudi riyal fell against the US dollar in the forward foreign exchange market on Thursday after the bill was passed, prompting a drop in dollar demand in Riyadh.

The law grants an exception to the legal principle of sovereign immunity in cases of terrorism on

US soil, clearing the way for lawsuits by the families of victims of the attacks seeking damages from the Saudi government.

Riyadh denies longstanding suspicions that it backed the hijackers who attacked the United States in 2001. Fifteen of the 19 hijackers were Saudi nationals.

Barack Obama brands Congress decision on Saudi 9/11 lawsuits bill 'a mistake' https://t.co/eykGvwUH2K — The Independent (@Independent) 29 septembre 2016

Euronews correspondent Stefan Grobe said:

“It was the first override of Barack Obama’s presidency and a sharp setback for a long time US ally, Saudi Arabia. It also reflects the extent to which Saudi influence in Washington has waned…

“The tide is turning against Riyadh as one analyst put it. The glorious days of the Saudi-American friendship are over.”