Hurricane Irma weakened to a Category 1 storm early Monday as it marched up the US state of Florida's northwestern coast, with its eye located about 25 miles (40 kilometers) northeast of the vulnerable Tampa area.

Maximum sustained winds had decreased to near 85 miles per hour as of 2:00 am local time (0600 GMT), with Irma projected to become a tropical storm as it moved into northern Florida or southern Georgia later Monday.

Warnings of dangerous storm surges remained in effect through vast swaths of peninsular Florida, where more than six million people had been ordered to flee Irma in one of the biggest evacuations in US history.