"Frankly, Saudi Arabia has not treated us fairly, because we are losing a tremendous amount of money in defending Saudi Arabia," he said.
Mr Trump's criticism of Riyadh, the world's top oil exporter, was a return to his 2016 election campaign rhetoric when he accused the kingdom of not pulling its weight in paying for the US security umbrella.
"Nobody's going to mess with Saudi Arabia because we're watching them," Mr Trump told a campaign rally in Wisconsin a year ago. "They're not paying us a fair price. We're losing our shirt."
Saudi Arabia's powerful deputy crown prince Mohammed bin Salman met with Mr Trump last month in a meeting that was hailed by a senior Saudi adviser as a "historical turning point" in relations. The talks appeared to signal a meeting of the minds on many issues, including their shared view that Iran posed a regional security threat.
Riyadh and other Gulf allies see in Trump a strong president who will shore up Washington's role as their main strategic partner and help contain Riyadh's adversary Iran in a region central to US security and energy interests, regional analysts said.
Asked about the fight against ISIS, which Saudi Arabia and other US allies are confronting as a coalition, Mr Trump said the terror group had to be defeated.
"I have to say, there is an end. And it has to be humiliation," Mr Trump said, when asked about what the endgame was for defeating terrorism.
Donald Trump has set a more positive tone with Israel than his Democratic predecessor, Barack Obama, who often clashed with the Israeli leader, and has raised concerns among Palestinians that their leaders may not get equal treatment.
Mr Trump has also asked Israel to put unspecified limits on its building of Jewish settlements on land the Palestinians want for a state, and has promised to seek a Middle East peace deal that eluded his predecessors. However, he has offered no new diplomatic prescriptions.
"I want to see peace with Israel and the Palestinians," he said. "There is no reason there's not peace between Israel and the Palestinians - none whatsoever."
Trump brushed aside a question of whether he might use a possible trip to Israel to declare U.S. recognition of the entire city of Jerusalem as Israel's capital, a reversal of longstanding US foreign policy likely to draw international condemnation.
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