TOKYO - A powerful tropical storm lashed Japan Thursday, bringing strong winds and torrential rain that prompted warnings of landslides and flooding, and sparked evacuation advisories and travel chaos at a peak holiday period.

Severe Tropical Storm Krosa -- one notch below a typhoon -- was churning slowly just off the southwestern coast of Japan, packing wind gusts of up to 160 kilometers per hour.

Authorities issued a voluntary evacuation advisory to around 550,000 people in the storm's path and Japan's Fire and Disaster Management Agency said four people had sustained minor injuries with one person more seriously hurt.

The agency also said that a party of 18 people including children got stranded during a barbecue in a valley when the river rose rapidly. They have since been evacuated to higher ground and should be rescued later Thursday.

Krosa also sparked travel chaos as people returned to major cities following the Obon holiday.

More than 600 domestic flights were cancelled to and from cities in western Japan and bullet train services were either scrapped or sharply reduced.

Ferries connecting the southern Shikoku island and other parts of Japan were also cancelled as high waves lashed the coast.

Krosa weakened significantly from earlier in the week as it stalled in the Pacific Ocean but it boasts an unusually large eye, meaning it is likely to dump rain over a wide area.

It is also moving very slowly -- 20 kilometers per hour -- so the rain is expected to last for an extended period.