US President Donald Trump is directing federal agencies to help manage a sewage spill that has led to millions of gallons of wastewater flowing into the Potomac River, which runs through the nation's capital.

In a social media post on Monday, Trump ordered federal authorities to "immediately provide all necessary Management, Direction, and Coordination to protect the Potomac", saying local leaders were turning the river "into a Disaster Zone".

Part of a sewer line in Maryland collapsed in late January, creating what public health experts say is one of the largest sewage spills in US history.

The river originates in West Virginia and flows through Maryland and Washington to the Chesapeake Bay.

Trump said that going forward, the Federal Emergency Management Agency (FEMA), which is part of the Department of Homeland Security (DHS), will lead the response.