US President Donald Trump has said that the US has "no stronger partner" than Saudi Arabia during his highly anticipated first major foreign trip - a whirlwind through the Persian Gulf that is heavily focused on investment.

U.S. President Donald Trump signed a strategic economic agreement with Saudi Arabia on Tuesday as the oil power rolled out the red carpet for him at the start of a tour of Gulf states aimed at drumming up trillions of dollars in investments.

President Trump, who will also go to Qatar and the United Arab Emirates, has told advisers that he wants to sign deals worth more than $1 trillion on the trip.