NEW YORK (NYTI MES) - Hurricane Sam strengthened into a Category 4 storm as it moved west across the Atlantic Ocean, forecasters said on Saturday (Sept 25).

The hurricane was about 1,000 miles east-southeast of the northern Leeward Islands as of 5pm Eastern time on Saturday, moving about 10 mph, with maximum sustained winds of 140 mph, according to the US National Hurricane Centre.

The swells generated by the hurricane were forecast to reach the Lesser Antilles, a group of islands in the Carribbean Sea, early next week and have the potential to cause life-threatening surf and rip current conditions, the centre said.

Beyond the swells, it was "still too early to tell what impacts might occur in the United States", Dennis Feltgen, a meteorologist at the hurricane centre in Miami, said on Saturday.

Sam, which formed on Thursday in the central Atlantic, is the fourth named storm to develop in less than a week and the 18th overall in a busy 2021 Atlantic hurricane season.

"All folks need to do right now is check the latest forecast as we get into next week," Feltgen said. He said people should ensure they have a hurricane plan, including supplies, in place.

Also on Friday, Subtropical Storm Teresa formed north of Bermuda, becoming the 19th named storm of the hurricane season. Teresa has a small window to "intensify slightly", but it is likely to dissipate by Sunday and also is not expected to threaten land, forecasters said.