A dissident Saudi official has alleged that Crown Prince Mohammed bin Salman forged his father's signature in a royal decree which committed Saudi ground troops in the early stages of the Yemeni civil war.

Jabri said he had discussed the war, which began in September 2014, with Susan Rice, national security adviser to then-US President Barack Obama. Rice said that Washington would only support an air campaign.

"We were surprised that there was a royal decree to allow the ground interventions," Jabri told the BBC. "He forged the signature of his dad for that royal decree. The king’s mental capacity was deteriorating."

A Saudi-led coalition, which included the United Arab Emirates, intervened on behalf of the Yemeni government in March 2015 to push back the Houthis, after the group took control of Sanaa.