The island's 813-meter (2,683-foot) Mount Oyama volcano may already be erupting beneath the sea, experts said.

A patch of sea bubbled and turned a milky white shade off the western tip of Miyakejima island, home to about 3,900 people and lying about 150 kilometers (93 miles) southwest of Tokyo.

A rush of 2,533 tremors rattled the volcano in the first 13 hours of the day and were now increasing in frequency and strength, a Meterological Agency official said.

The strongest, measuring 4.0 on the open-ended Richter scale, struck at 11:44 a.m. (0244 GMT).

"There is a possibility of a strong explosion near the shore," Tsutomu Takeuchi, head of the Meteorological Agency's volcanic division, told a news conference.

"Also, we cannot rule out an eruption on the western side of the mountain," he said. The volcano which forms the island last erupted in October 1983.

Authorities moved 2,144 people from their homes on the eastern and western sides of the volcano to shelter in a school gymnasium and other public buildings in the safer northern area, local police said.

Navy sailors practiced evacuation procedures with rubber dinghies and landing craft on a beach near the island's northern Okubo port, an AFP photographer said.

Eight destroyers, two mine-sweeper escort ships, a transport ship, fuel ship and a submarine salvage vessel, with total capacity for more than 9,750 passengers, stood by.

"We may have to pick up the residents by ships," Maritime Self-Defense Force spokesman Yoshikazu Hirose said.

A naval transport ship delivered fire-fighting equipment.

Three helicopters and a private ship moved in food, blankets and a medical team on the orders of the Tokyo metropolitan government, which is responsible for the island.

All four commercial flights were cancelled between Tokyo and Miyakejima, an island formed out of the volcano which has become a popular weekend diving spot.

"We are just feeling occasional tremors," said Rie Noda, a 33-year-old worker at the Snapper Diving Center in Kamitsuki on the northern tip of the island.

"But we don't think expect anything terrible like having our homes buried under lava," she told AFP.

The government set up a crisis-management centre at the prime minister's residence to coordinate reactions.

"The local town, the Tokyo government and the central government are closely cooperating," Prime Minister Yoshiro Mori told reporters.

"We have to very carefully monitor the situation."

The volcano erupts roughly every 20 years. When it last blew in 1983, about 500 homes were buried by lava on the western side of the island but there were no injuries.

The 2,000-metre-high (6,600-foot-high) volcano began erupting Friday, sending fountains of molten rock into the sky, and lava flows creeping down its flanks.

Hundreds of people gathered along Reunion's coast overnight Saturday to admire the spectacle.

By Sunday, the lava had flowed down to an altitude of 350 metres and was only one kilometre from the island's main road which runs along the east coast.

But the flows were slowing as they reached more level ground, RVO director Thomas Staudacher.

The observatory registered new seismic activity Saturday and did not rule out another eruption from the mountain's Dolomieu crater. It was also carefully monitoring strong seismic activity in other parts of the island.

In 1977 and 1986 Reunion's inhabitants had to be evacuated as eruptions destroyed homes and crops.