At around 2100 GMT, Franklin was about 855km southwest of Bermuda with maximum sustained winds nearing 155kph, the Miami-based centre said in an advisory.

“Additional strengthening is forecast, and Franklin is forecast to become a major hurricane on Monday,” the advisory said.

A tropical storm with maximum sustained winds of 177kph or higher is considered a major hurricane.

Franklin is expected to turn away from the US eastern seaboard in the early part of the week but pass near the island of Bermuda on Wednesday, an NHC map shows.

The NHC said swells generated by Franklin were expected to begin affecting Bermuda on Sunday night.

“These swells are also likely to cause life-threatening surf and rip current conditions through the beginning of this week along portions of the east coast of the United States,” it said in the advisory.

This year’s Atlantic hurricane season, which runs from June 1 to Nov 30, is expected to have 18 named tropical storms, nine of which become hurricanes, four of them major, according to forecasters from Colorado State University.

A separate storm, Idalia, is expected to become a hurricane and could cause a life-threatening storm surge and flooding from heavy rains along parts of Florida’s west coast and the Panhandle as early as Tuesday, according to the NHC.