Hurricane Franklin gains strength, takes aim at Bermuda

"Additional strengthening is forecast, and Franklin is forecast to become a major hurricane on Monday," the centre said.

A tropical storm with maximum sustained winds of above 177km/h or higher is considered a major hurricane.

Franklin is expected 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 November 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.