(Adds issue of emergency warnings, details)
By Elaine Lies
TOKYO, July 7 (Reuters) - Japan's weather agency on Monday issued emergency warnings to urge people in the country's southern islands to take maximum precautions as a super typhoon described as a "once in decades storm" is set to rake the Okinawa island chain with heavy rain and powerful winds.
Typhoon Neoguri was already gusting at more than 250 km an hour (150 mph) and may pick up still more power as it moves northwest, growing into an "extremely intense" storm by Tuesday, the Japan Meteorological Agency (JMA) said.
But it was not expected to be as strong as Typhoon Haiyan, which killed thousands in the Philippines last year.
The JMA issued emergency storm and high sea warnings for Japan's small southern island of Miyakojima, some 300 km (188 miles) southwest of Okinawa island, and for a smaller nearby islet.
The agency said on Monday evening it also planned to issue an emergency high sea warning for Okinawa island, host to three-quarters of U.S. military facilities in Japan.
"In these regions, there is a chance of the kinds of storms, high seas, storm surges and heavy rains that you've never experienced before," a JMA official told a news conference.
"This is an extraordinary situation, where a grave danger is approaching."
The storm was south of Okinawa but moving northwest at 25 kph (16 mph) with sustained winds of 180 kph (110 mph) by 7:00 p.m. (1000 GMT), the JMA said on its web site.
The JMA official urged people in the target areas to evacuate early and take precautions. Television showed fishermen winching their boats out of the water.
There are no nuclear plants on Okinawa, but there are two on Kyushu, Japan's westernmost main island that lies in the area through which the typhoon is likely to pass, and one on Shikoku island, which borders Kyushu and could also be affected.
All are halted in line with current national policy. A spokeswoman at Kyushu Electric Power Co said there were no specific plans related to this typhoon but the company had plans in place year-round to protect the plants from severe weather.
The Fukushima Daiichi nuclear plant, crippled by an earthquake and tsunami in 2011, is on the other side of the country, which is likely to see rain, at the worst.
Keiji Furuya, state minister in charge of Disaster Management, cancelled a planned trip to the United States.
The commander at Kadena Air Base, one of the largest U.S. military establishments on Okinawa, warned that damaging winds were expected by early Tuesday.
"I can't stress enough how dangerous this typhoon may be when it hits Okinawa," Brigadier General James Hecker wrote on the base's Facebook page on Sunday. "This is not just another typhoon."
Around two to four typhoons a year make landfall in Japan but they are unusual in July.
(Reporting by Elaine Lies and Kiyoshi Takenaka,; Editing by Nick Macfie)