Trump signs business deals worth $380 billion in Saudi Arabia

President Donald Trump managed to secure $380 billion worth of business from Saudi Arabia, including $110 billion for US defence firms, on his first overseas outing, as he played up the threat posed by Iran and Islamic terrorism to the Islamic world.

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 elsewhere in Europe.

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.

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."

Addressing the Arab-Islamic-American Summit, Trump praised Saudi Arabia's leadership of the Islamic world even as Saudi ruler King Salman said Trump's effective participation in the summit reflects his keenness on enhancing cooperation and coordinate stances in various fields.

''The meeting significantly shows that our 55 Arab and Islamic countries attending the summit today, with a population exceeding 1.5 billion, are an important partner in fighting the forces of extremism and terrorism and achieving world peace, security and stability,'' said the king.

Iranian foreign minister Mohammad Javad Zarif tweeted on Sunday that the United States may be "milking" Saudi Arabia of $380 billion after Washington signed major deals with Tehran's neighbour.

However, the Saudi ruler termed the business deals signed with the US as constructive cooperation to renounce extremism and work on countering terrorism in all its forms, drying up its sources, and stopping all means of financing and disseminating terrorism.

President Donald Trump is on an eight-day trip that will also take him to Jerusalem, the Vatican and elsewhere in Europe, as he looks to leave domestic troubles behind and boost business sentiment in the US.

Secretary of State Rex Tillerson said the arms deal was intended 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 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.

Trump is looking at ways to effectively tap the divergence between Sunni power Saudi Arabia and predominantly Shiite Iran that are on either side of a range of regional conflicts, including in Syria and Yemen, where Riyadh is leading a military coalition battling Tehran-backed rebels.

Tillerson said the arms package "bolsters the kingdom's ability to provide for its own security and contributing to counter-terrorism operations throughout the region".

For Trump, the mood in Riyadh was also in sharp contrast to Washington where pressure is building after fresh claims that James Comey, the former FBI chief fired by Trump, had agreed to testify publicly about Russian interference in the US elections.

Reports also emerged that Trump had called Comey "a nut job" and that the FBI had identified a senior White House official as a "significant person of interest" in its probe of Russian meddling.