The CEO of Saudi Arabia's national oil company warned Monday of a coming global oil shortage that U.S. shale oil supplies and fracking won't be able to stop.

"If we look at the long-term situation of oil supplies, for example, the picture is becoming increasingly worrying," said Amin Nasser, CEO of Saudi Aramco, the world's largest oil company, speaking Monday at a conference in Istanbul.

Nasser said the biggest problem affecting supply in the next few years will be the current drop in fossil fuel investment and exploration, adding that shale oil development in the U.S. and elsewhere won't happen fast enough to help the world avoid a shortage.

"Financial investors are shying away from making much-needed large investments in oil exploration, long-term development and the related infrastructure," he said, according to Reuters. "Investments in smaller increments such as shale oil will just not cut it."

The comments come after President Trump visited Europe, touting his policy of "energy dominance" that looks to promote the U.S.'s oil and natural gas supply boom. Trump wants to boost oil and gas production on federal lands and begin exporting more of its fossil fuels abroad.

Saudi Arabia, which relies heavily on energy production for its kingdom's revenue and is trying to diversify, is planning to sell 5 percent of Aramco in an initial public offering next year.

Nasser said $1 trillion in oil and natural gas investment has been lost in the wake of the global oil glut that saw prices drop dramatically in 2014. Recent projections show that 20 million barrels per day of new oil production will be needed in the next five years to meet growing demand and offset natural decline in older oil fields, he said.

"New discoveries are also on a major downward trend. The volume of conventional oil discovered around the world over the past four years has more than halved compared with the previous four," Nasser said.