Last year Trump made Saudi Arabia his first stop on his maiden worldwide trip as a president.
Trump did not specify when he made the remarks to the Saudi monarch but they come on the back of increasing oil prices in the US.
Brent crude rose almost 2 percent after hitting a four-year high on Wednesday as the market focused on upcoming USA sanctions on Iran while shrugging off the year's largest weekly build in US crude stockpiles and reports of higher Saudi Arabian and Russian production.
President Donald Trump said the king of Saudi Arabia may not last two weeks without USA support, escalating pressure on one of his closest Arab allies to curb rising oil prices and pay for military protection. "You have to pay for your military, you have to pay'".
"We protect Saudi Arabia - would you say they're rich?"
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Trump didn't elaborate on when he spoke to the king.
Benchmark Brent crude oil is near $85 a barrel - a four-year high - and analysts say it could reach $100.
Saudi Arabia now produces some 10 million barrels of crude oil a day. Trump said, in his -N General Assembly speech in NY last month, that OPEC members were ripping off the rest of the world. This is predicted to show that weekly crude inventories rose by 1,985,000 barrels after the previous week's 1,852,000 increase.
The US crude oil price is pausing for breath after its strong advance ahead of US sanctions on Iran, which take effect on November 4. South Korea and Japan also have the USA to thank for stability in the Asian region, according to Trump, who previously demanded increased payments from United States allies to keep American bases in their countries. "We want them to start lowering prices and they must contribute substantially to military protection from now on".
Trump, facing political pressure at home, has been calling on OPEC and allies like Saudi Arabia to boost their production to lower global crude oil prices.
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.