RIYADH -  Agreements worth more than $380 billion have been signed between Riyadh and Washington on the first day of US President Donald Trump's visit, the Saudi foreign minister said on Saturday.

"The two countries signed a series of agreements," Adel al-Jubeir told reporters at a joint press conference with US Secretary of State Rex Tillerson. He said the "total value of investments... (is) in excess of $380 billion", including by Saudi Arabia in American infrastructure, and US investment in developing the kingdom's defence industry.

"We expect that these investments over the next 10 years or so will provide hundreds of thousands of jobs in both the United States and in Saudi Arabia," Jubeir said.

Tillerson clarified that almost a third of the total figure is military-related.

Separately, Saudi Arabia's state oil company Saudi Aramco said in a statement that it had signed joint venture and MOU agreements with American companies estimated to be worth about $50 billion.

Washington also announced a huge arms deal with Saudi Arabia as President Donald Trump began a foreign tour looking to leave domestic troubles behind.

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. "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.

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.

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".

The harder line on Iran will be very welcome in Saudi Arabia and among its Arab Gulf allies, who saw Trump's predecessor Barack Obama as too soft on Tehran.Trump had been assured a warm welcome in Riyadh, which is keen to rebuild ties with a major ally.

The president and first lady Melania Trump were greeted by King Salman as they disembarked at King Khalid International Airport in Riyadh on Saturday morning.

Trump and his wife, who dressed conservatively in black but did not cover her hair as Saudi women are required to do, walked side-by-side to the tarmac where they both shook hands with the 81-year-old king.

Trump in 2015 criticised then-first lady Michelle Obama for not wearing a headscarf during a visit to Saudi Arabia, saying on Twitter that her hosts had been "insulted". Later, Trump joined in a traditional sword dance welcome ceremony ahead of a banquet at the Murabba Palace.

After talks with senior Saudi officials, Trump was to give a speech to dozens of Muslim leaders on Sunday. It has been touted as a major event - along the lines of a landmark address to the Islamic world given by Obama in Cairo in 2009.

It will be especially sensitive given tensions sparked by the Trump administration's attempted travel ban targeting several Muslim-majority nations and accusations of anti-Islamic rhetoric on the campaign trail.

While most US presidents make their first foreign trip to neighbouring Canada or Mexico, 70-year-old Trump has opted instead for the Middle East and Europe.

The first leg of the trip was likely to be the easiest -- Saudi leaders are keen to work with an administration they see as more in line with their goals. The normally austere kingdom put on a major display for the visit, with Riyadh's streets lined with US and Saudi flags and billboards featuring Trump and King Salman.

The next leg in Israel could be more complicated, despite the history of warm ties between Trump and Prime Minister Benjamin Netanyahu.

The visit is already awash in controversy, from a row over Trump's planned visit to Jerusalem's Western Wall, the holiest prayer site for Jews, to his alleged disclosure of Israeli intelligence to Russian officials. After meeting Netanyahu in Jerusalem, Trump will see Palestinian president Mahmud Abbas in Bethlehem, part of his efforts to revive the long-stagnant peace process.

On Tuesday Trump heads to Rome to meet with Pope Francis. The two men are at odds on everything from climate change to refugee policy, although the pontiff says he will give the US leader an open-minded hearing.

The president will then meet NATO members in Brussels and attend a G7 summit in the Sicilian town of Taormina.

These meetings - including with new French President Emmanuel Macron - will be closely watched for signs of whether Trump and traditional US allies in Europe can work together.