US President Donald Trump said on Wednesday he was not happy with planemaker Boeing and his administration might have to go a different route with Air Force One planes.

The comments were the White House's latest attempt to ratchet up pressure on Boeing, which is at least three years behind schedule in delivering two new Air Force One jets.

Trump and the White House have recently railed against the jet maker, saying it has failed to deliver a new aircraft on time.

"I'm not happy with Boeing," Trump told reporters on Air Force One, airing his frustration once again. "We gave that contract out a long time ago."

The fixed-price contract was taking too long, he said, adding, "We may do something else. We may go and buy a plane, or get a plane or something."

But he ruled out Boeing's rival Airbus as an alternative.

There was no immediate comment from Boeing.

On Tuesday, a White House official said Boeing's Air Force One program could be delayed until 2029, or beyond, citing supply chain issues and changing requirements, after White House remarks over the weekend that it had fallen behind.

The delays are frustrating, but little can be done to speed delivery, the official told Reuters, adding that Boeing faced problems after the makers of some components went out of business.

Trump has been deeply engaged with the program since his 2016 presidential campaign.

He extracted a promise from then-CEO Dennis Muilenburg to cap the program's cost at $4 billion. Those fixed-price contract terms, questioned by analysts at the time and finalised in 2018, have cost Boeing more than $2 billion.

Trump's renewed engagement could signal further problems for Boeing, which said the company was meeting his billionaire cost-cutting ally Elon Musk to get the plane updated quicker, according to analysts.