TOKYO, Aug 3 (Reuters) - A sharply weakened Typhoon Usagi churned north over the Sea of Japan on Friday after cutting across Japan's southernmost main island of Kyushu, injuring 18 people.

Usagi, the second large storm to hit Japan in less than a month, was downgraded to a tropical storm by international standards, but forecasters warned that it could still bring heavy rain to Niigata prefecture in northwestern Japan, which was struck by a deadly earthquake last month.

The storm was roughly 10 km (6 miles) northwest of the city of Izumi, some 600 km (375 miles) west of Tokyo, as of 9:00 a.m. (0000 GMT), the Japan Meteorological Agency said.

It had winds gusting up to 126 km an hour (78 mph) and was heading north at 30 km an hour (19 mph).

"The storm is expected to curve east and may bring heavy rain to Niigata on Saturday, so people have to be alert for the chances of landslides and other damage," a Meteorological Agency official said.

One of the injured was a 42-year-old man who sustained head injuries after falling from a roof.

Last month, Typhoon Man-yi struck Kyushu and killed three people, injured more than 70 and flattened several houses.

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