The $110 billion deal for Saudi purchases of US defence equipment and services came at the start of an eight-day trip that will also take Trump to Jerusalem, the Vatican and meetings with leaders in Europe.

Trump hailed a series of business deals reached during meetings in Riyadh, with Saudi Foreign Minister Adel al-Jubeir saying they were worth more than $380 billion.

"That was a tremendous day. Tremendous investments in the United States," Trump said at talks with Saudi King Salman.

"Hundreds of billions of dollars of investments into the United States and jobs, jobs, jobs."

White House spokesman Sean Spicer said on Twitter that the defence agreement was the "largest single arms deal in US history" and said other deals amounted to $250 billion in commercial investment.

Secretary of State Rex Tillerson said the intent of the arms deal was to support Riyadh "in particular in the face of malign Iranian influence and Iranian-related threats which exist on Saudi Arabia's borders".

Tillerson also urged Iran's President Hassan Rouhani, who won a resounding re-election victory on Saturday, to dismantle his country's "network of terrorism" and to end ballistic missile tests.

Sunni power Saudi Arabia and predominantly Shiite Iran are opposed in a range of regional conflicts including in Syria and in Saudi neighbour Yemen, where Riyadh is leading a military coalition battling Tehran-backed rebels.