RIYADH (Reuters) -BlackRock CEO Larry Fink says tens of trillions of dollars of financial firepower is sitting idle in cash amid trade war worries and uncertainty over the United States economy, whose deficits are beginning to spook investors.

"There is 12 trillion euros sitting in bank accounts in Europe. In the United States, there's $11 trillion sitting in money markets funds. When there is uncertainty, you are going to keep more and more money in cash and that is what we witnessed," Fink told delegates at the Saudi-US Investment Forum in Riyadh on Tuesday.

The veteran financier, who runs the world's largest asset manager, made the comments shortly after U.S. President Donald Trump arrived in Saudi Arabia to kick off a four-day tour of the Gulf region.

Besides the kingdom, Trump will be taking in Qatar and the United Arab Emirates, with the spotlight on economic deals as well as various security crises ranging from war in Gaza to the threat of escalation over Iran's nuclear program.

Fink said many global investors were still overweight on the U.S. despite market ructions caused by Trump's proposed revamp of global trade ties. A "modest reallocation" away from U.S. assets had benefited Europe and stoked increasing investor interest in the Gulf region, India and Japan, he said.

But continued confidence was not guaranteed, with another 90 days of volatility in prospect following Trump's latest trade tariff hiatus with rival economic superpower China.

"I think we are going to be in a period of uncertainty and we are going to start focusing on the role of deficits, which are not part of any conversations," he told the event, which was also attended by U.S. Treasury Secretary Scott Bessent and Saudi Finance Minister Mohammed Al Jadaan and Investment Minister Khalid al-Falih.

"Let's be clear, the U.S. deficits are an issue ... the US economy needs a 3% growth rate to overcome its deficits and I do believe what President Trump is trying to do is consistent with what the kingdom has been trying to do, they are trying to create more public-private investing," Fink said.

He also praised Saudi Arabia's Vision 2030 economic transformation program on Tuesday, saying the kingdom was building a diversified economy that had potential to become a 21st century global leader.

Amid lower oil prices and rising costs, some of the kingdom's lofty ambitions for Vision 2030, including a futuristic city in the desert, have been scaled back to prioritise projects essential to hosting global sporting events over the next decade.

Stephen Schwarzman, CEO of private equity firm Blackstone, alluded to some of the teething problems seen in the Crown Prince Mohammed Bin Salman's program, but urged the country to remain on track.

"You are going to accomplish a lot of these things but like all great visions some of them wont happen and you can't get discouraged because that is nature of major change," he said at the event.

(Reporting by Yousef Saba, Nafisa Eltahir, Pesha Magid, Federico Maccioni in Riyadh; Writing by Sinead Cruise in London. Editing by Elisa Martinuzzi.)