SYDNEY, Dec 18 (Reuters) - Fiji's government warned residents to stay alert on Friday as a slow-moving hurricane ground across the Pacific island nation, bringing gusts of wind of up to 285 km per hour (177 miles) and causing flooding and landslides.

The powerful Cyclone Yasa, a top category five storm, landed over the Bua province on the northern island of Vanua Levu on Thursday evening, bringing torrential rain and widespread flooding across low-lying areas of the archipelago.

"#TeamFiji, we are not out of the woods yet, keep safe and adhere to weather warnings!!" the office of Prime Minister Frank Bainimarama posted on social media on Friday morning.

Ahead of a nationwide curfew overnight on Thursday, the prime minister had warned the country's near 1 million population to find shelter ahead of one of the southern hemisphere's most intense tropical storms on record.

Images shared on social media showed roads blocked by landslides, floodwaters and fallen trees. All roads in Rakiraki, a district on the main island with about 30,000 residents, were flooded, according to local media.

"Because the cyclone kind of lingered over Vanua Levu for almost four hours we expect the destruction to be quite severe," Shairana Ali, CEO of Save the Children in Fiji told the Australian Broadcasting Corporation on Friday morning.

"Some of the evacuation centres, especially the schools in the Bua district, the roofs had blown away so people had to basically move from classroom to classroom at the height of the cyclone to take shelter."

The cyclone, which is moving southeast at about 22 km per hour, was passing through smaller islands around Lomaiviti on Friday morning. (Reporting by Paulina Duran in Sydney; Editing by Alistair Bell)