President Donald Trump's call to spend $1 trillion updating the nation's infrastructure systems is missing the key element of how to pay for it, former Federal Reserve Chairman Alan Greenspan told CNBC on Thursday.
"The question is who is financing this trillion dollars in infrastructure," Greenspan said on "Squawk on the Street," from the sidelines of the spring meeting of the IMF and the World Bank in Washington.
"At the moment, we can't afford it," he said, arguing the U.S. has "too much debt."
Infrastructure projects have too many upfront costs, Greenspan said. "Infrastructure-type of spending is of delayed impact."
"If you go out and build a bridge, for example, you don't get any revenues from that bridge until it's complete," he explained.
The White House has said it plans to leverage public-private partnerships to defray costs for the president's infrastructure plans. But most of the details on how to pay for them have not been hashed out yet.
Greenspan told CNBC on Thursday that the best way to pay for cutting taxes is to reduce spending on ballooning entitlements such as Social Security and Medicare.
Starting in 1987, the 91-year-old economist led the Fed for 19 years under four presidents, from Ronald Reagan through George W. Bush.