US President Donald Trump received Saudi Arabia's highest civilian honour, as he embarked upon the first day of his visit to the long-time US ally.

The Collar of Abdulaziz al-Saud was placed around Trump's neck by King Salman during a ceremony at the Royal Court in Riyadh.

During the event, it was announced that Trump was being honoured for "his quest to enhance security and stability in the region and around the world".

The honour has also been bestowed to Russian President Vladimir Putin, British Prime Minister Theresa May and Trump's predecessor, Barack Obama.

Trump was joined by first lady Melania Trump and several senior White House aides who were interspersed with Saudi officials throughout a grand ballroom.

Saudi Arabia and the US have agreed to a huge arms deal to bolster the kingdom's long-term security against Iranian threat.

"This package of defence equipment and services support the long-term security of Saudi Arabia and the Gulf region in the face of Iranian threats," a White House official said.

The US official described the agreements as "a significant expansion of the over seven-decade-long security relationship between the United States and the Kingdom of Saudi Arabia".