UAE President Mohamed bin Zayed's direct outreach to Saudi Arabia's elderly King Salman has been viewed by US and Arab diplomats as an effort to bypass the kingdom's powerful Crown Prince Mohammed bin Salman.

Several Saudi and Emirati news outlets reported on Monday that the leader of the UAE sent a handwritten letter to Saudi Arabia's 90-year-old King Salman.

Saudi news sites reported that the letter addressed the status of ties between the two Gulf neighbours, while the UAE's official news agency, WAM, said the letter was a personal invitation to Abu Dhabi to attend a UN-sponsored water conference in December.

Gulf watchers said the letter reflected the delicate ties between the Saudi crown prince and his Emirati counterpart.

"The Emiratis felt they had no other option than to go straight to the top," one Western official familiar with the matter told Middle East Eye.