The comments at a rally in Mississippi come just days after president complained that Washington was 'subsidising' Saudi Arabia's military
US President Donald Trump has claimed that he told Saudi Arabia's King Salman he would not last in power "for two weeks" without the backing of the US military.
"We protect Saudi Arabia. Would you say they're rich? And I love the King, King Salman. But I said 'King - we're protecting you - you might not be there for two weeks without us - you have to pay for your military,'" Trump said to cheers at a rally in Southaven, Mississippi.
Trump, speaking on Tuesday, did not say when he made the remarks to the Saudi monarch, the Reuters news agency reported.
The comments come just days after Trump complained that Washington was "subsidising" Saudi Arabia's military, echoing similar jibes at European members of the NATO alliance.
Speaking at a West Virginia rally for local candidates of his Republican Party on Saturday, the president also criticised Japan and South Korea over the same issue.
"When you have wealthy countries like Saudi Arabia, like Japan, like South Korea, why are we subsidising their military?" asked Trump, who pushes an aggressive "America first" policy on trade.
"They'll pay us. The problem is nobody ever asks."
Trump added that he had spoken to Saudi Arabia's King Salman on Saturday to make the same point, the AFP news agency reported.
Despite the harsh words, the Trump administration has had a close relationship with Saudi Arabia, which it views as a bulwark against Iran's ambitions in the region.
Trump made Saudi Arabia his first stop on his maiden international trip as president last year.
In the same call on Saturday, Trump and the king discussed efforts being made to maintain supplies to ensure oil market stability and global economic growth, according to Saudi state news agency SPA.
Saudi Arabia is the world's top oil exporter and the de facto leader of OPEC, which has been criticised by Trump for high oil prices.
Speaking at the United Nations General Assembly in New York last month, Trump said OPEC members were "as usual ripping off the rest of the world".
"We defend many of these nations for nothing, and then they take advantage of us by giving us high oil prices. Not good. We want them to stop raising prices, we want them to start lowering prices," Trump said.