WASHINGTON (REUTERS) - US President Joe Biden will unveil the first detailed budget proposal of his term in office on May 27, a spokesman said, offering a window into his priorities from defence spending to deficit reduction.

Biden in April outlined a glimpse at plans for about US$1.5 trillion in "discretionary" spending, which represents only part of the budget.

The full budget, which covers the fiscal year starting in October, will be the first exhaustive list of the programmes Biden wants to expand or cut - from foreign aid to immigration and policing. Rob Friedlander, a spokesman for the Office of Management and Budget, confirmed the late-May release date.

As the economy stumbles to recover from the coronavirus pandemic, the budget will offer a sense of how much the government wants to rein in deficit spending, as well as how it sees taxes and economic growth helping its agenda.

But it's merely one step in a spending process as Biden works to muscle through Congress some US$4 trillion in spending on proposals ranging from infrastructure to childcare. Lawmakers must sign off on any spending Biden requests.

The outline Biden released in April marked a sharp departure from his Republican predecessor Donald Trump, spreading billions across areas ranging from public transit, poor schools, toxic site clean-ups, and background checks on gun sales, but nothing on border walls.

But Biden's plans for the Department of Defence drew the most attention. Liberals were disappointed that the roughly-even funding on an inflation-adjusted basis at US$715 billion would fail to cut wasteful Pentagon spending. Conservatives want more spending to counter threats from China, Russia, Iran and North Korea.