MIAMI, United States (AFP) —The city of Jacksonville in northeast Florida ordered urgent evacuations on Monday as record floods from monster storm Irma were set to rise even higher with the oncoming high tide.

“If you are in Evacuation Zone A or B along the river you need to heed this warning and get out now,” the Jacksonville Sheriff’s Office said on its Facebook page Monday morning, referring to neighbourhoods along the St Johns River which bisects the city.

“Historic flood levels already. Levels will continue to rise. Expect 4-6 feet (1.2-1.8 metres) above normal high tide levels. Very significant river flooding is coming. It is going to be bad at high tide around 2:00 pm.”

Residents who needed help getting out were urged to put a white flag, T-shirt, or other white object in front of their home to grab the attention of search and rescue teams.

“In the event of rising water, go up, not out,” the sheriff’s department added.

Video posted on Twitter Monday morning showed murky floodwater flowing quickly along a street in downtown.

Jacksonville, population 880,000 was not in the direct path of Irma, which was downgraded to a tropical storm earlier Monday and travelling north roughly in line with the state’s western coast. But that placed Jacksonville in the northeast portion of the massive storm, putting it at the greatest risk of powerful winds and rain, with Irma’s tropical storm-force winds extending out 415 miles (665 kilometres) from the eye.