Miami, Florida - Ernesto grew into a hurricane Wednesday, US forecasters said, after leaving more than 600,000 customers without power in the US territory of Puerto Rico.

The storm brushed past the Caribbean island overnight, and was expected to dump up to 10 inches of rain in some places, the US National Hurricane Center (NHC) said.

It became a Category 1 hurricane on the five-category Saffir Simpson scale on Wednesday morning, with maximum sustained winds of 75 miles per hour, the NHC said.

The storm left some 604,000 customers in Puerto Rico without electricity, according to power company Luma Energy.

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The Puerto Rican power grid has suffered frequent outages since Category 4 Hurricane Maria tore through it in 2017.

The Canovanas River, in the northeast of the island, burst its banks due to heavy rainfall, according to Telemundo, which reported a risk of more flooding.

The NHC said the nearby Virgin Islands could also see "significant flooding." Both the Virgin Islands and Puerto Rico remained under a tropical storm warning.

Ernesto was expected to turn north into the Atlantic Ocean later Wednesday, with nothing but 800 miles of warm open water between it and the tiny British territory of Bermuda, which the NHC said it could near by Friday or Saturday.

The NHC said it may strengthen into a major storm – Category 3 or higher – in the coming days.

Scientists say climate change likely plays a role in the rapid intensification of storms because there is more energy in a warmer ocean for them to feed on.

Warmer than normal Atlantic waters threaten to supercharge this year's hurricane season, scientists have warned.