Search and rescue teams in Florida are due to swing into action as daylight reveals the extent of damage wreaked by Hurricane Irma.

The huge storm weakened as it moved up the state’s western coast overnight but Miami and other urban areas have been battered and inundated. The emergency manager in Monroe County, which includes the Keys, has warned of a looming “humanitarian crisis”.

Media reports link at least four deaths to the storm.

Irma is now a category one hurricane but still has maximum sustained winds of 75mph (120km/h). It cut a devastating track across 10 Caribbean countries and territories, killing at least 28 people there.

More than 3.4 million homes in Florida are without power. Power lines came down in 80% of Miami alone and large areas of the city were flooded. Drone footage from Naples, a town on the coast of the Gulf of Mexico about 125 miles (200km) north-west of Miami, shows rows of shattered suburban homes on streets under water.

President Donald Trump has approved a major disaster declaration and emergency federal aid for Florida, describing the hurricane as a “big monster”. Federal funds will be needed to care for victims, clean up debris, restore power, and repair damage to homes and businesses.

Martin Senterfitt, emergency management director for Monroe (population 73,000), said a huge airborne relief mission mounted by the Air Force and Air National Guard was in the works, the Miami Herald reports. “Disaster mortuary teams”, he said on Sunday afternoon, would be dispatched to the Keys, which are a chain of low-lying islands.

Florida Governor Rick Scott continued to tweet storm warnings early on Monday. “As little as six inches [15cm] of moving water can knock you down,” he said in one message. “Stay inside. Stay safe.”