US President Donald Trump on Monday declared an emergency in Louisiana as part of the southern state was being pelted with rain from tropical storm Harvey.

The declaration allows the federal government, specifically the Federal Emergency Management Agency (FEMA) to coordinate relief efforts. It also provides federal funding for disaster relief.

"This action will help alleviate the hardship and suffering that the emergency has inflicted on the local population," the White House said in a statement.

"Specifically, FEMA is authorized to identify, mobilize, and provide at its discretion equipment and resources necessary to alleviate the impacts of the emergency."