US President Donald Trump says his long-awaited plan to help rebuild the nation's infrastructure would result in about $US1.7 trillion ($A2.4 trillion) in overall investment over the next 10 years, a larger figure than he previously announced.

The plan, which will be detailed in part at next week’s State of the Union address, “will actually probably end up being about $US1.7 trillion,” Trump told a gathering of mayors at the White House on Wednesday.

Trump previously valued the plan, which is expected to feature a mix of federal and local investment, at $US1 trillion.

The administration is expected to ask Congress for $US200 billion in federal spending on infrastructure aimed at encouraging more than $US1 trillion in state, local and private financing to build and repair the nation’s bridges, highways, waterworks and other infrastructure.

Republican Senator John Thune, who chairs the Commerce Committee, told Reuters in an interview on Tuesday that the big challenge “would be how do you pay for it and the (Trump administration is) talking about spending cuts, which are unspecified.”

Thune said it was probably going to be “tough” to get Democrats to support a deal without the federal government raising additional revenue for improvements.

He said in order to get a bill approved this year, it would need to be moving through both the House of Representatives and Senate by late spring or early summer. Trump will need to make a big push to win approval, Thune added.

Reuters reported last week that the plan involved $US100 billion in cost-sharing payments for projects and $US50 billion for rural projects, with the remaining $US50 billion largely split among “transformative” projects such as high-speed trains, and funds for federal transportation lending projects.

A leaked document released this week, which an administration official confirmed was “largely accurate,” shows that the administration plans to reduce the cost-sharing for projects to no more than 20 per cent of the costs from the traditional 80 per cent federal share. That would result in a higher overall infrastructure boost if states agree to shoulder more of the costs.

Democrats have criticised that proposal and argued the Trump administration should back more direct federal spending.

The bill will also aim to streamline environmental reviews and it make it easier to build highways and other projects and allow for greater tolling on roadways.