Washington DC - Tech titan Elon Musk cast doubt Wednesday on a $500 billion AI project announced by President Donald Trump, saying the money promised for the investment actually wasn't there.

The comments marked a rare instance of a split between the world's richest man and Trump, with Musk playing a key role in the newly installed administration after spending $270 million on the election campaign.

In his first full day in the White House, Trump on Tuesday announced a major investment to build infrastructure for artificial intelligence led by Japanese giant SoftBank and ChatGPT-maker OpenAI.

Trump said the venture, called Stargate, "will invest $500 billion, at least, in AI infrastructure in the United States."

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But in a post on his social media platform X, Musk said the main investors "don't actually have the money."

"SoftBank has well under $10B secured. I have that on good authority," Musk added in a subsequent post.

Musk's sideswipe could be particularly targeted at OpenAI, the world's leading AI startup that Musk helped found, before leaving in 2018.

The Tesla boss and OpenAI chief executive Sam Altman, who was present at the White House on Tuesday, have been mired in a serious feud, with Musk opening repeated lawsuits against the company behind ChatGPT.

"Wrong, as you surely know. Want to come visit the first site already under way?" Altman replied to Musk on X.

"This is great for the country. I realize what is great for the country isn't always what's optimal for your companies, but in your new role I hope you'll mostly put [country] first," he added.

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OpenAI is one of the world's highest valued startups, but loses money on the high costs of turning out its expensive technology.

According to the Wall Street Journal, cloud giant Oracle, which is also involved, has about $11 billion in cash and securities. SoftBank has roughly $30 billion of cash on hand.

The Stargate project is committed to invest an initial $100 billion in the project, and up to $500 billion over the next four years.

Abu Dhabi's AI-focused state fund MGX and Oracle are also providing funding for the project, while SoftBank-owned Arm, Microsoft and Nvidia will be technology partners.

According to the companies, the project is initially building a data center operation in Texas, where construction is already underway.

Ahead of taking office, Trump this month unveiled a $20 billion Emirati investment in US data centers as well as a previous investment pledge from SoftBank.