Famously crowded Japan is getting a bit more space as a newly-formed volcanic island just keeps on growing.

New footage of the remote Nishinoshima, some 1,000 kilometres (620 miles) south of Tokyo, shows a volcano erupting up to six times a minute, spewing huge volumes of magma -- and scientists say there is plenty more still to come.

A tiny islet emerged in November 2013 right next to the original Nishinoshima, when molten rock cooled and began to poke its head just above the water.

That speck of land grew as the volcano kept going, and soon engulfed its once larger neighbour.

The new super-island is now a respectable 2.46 square kilometres (0.95 square miles), the Japan Coast Guard says -- roughly the size of 345 football pitches -- while the still-spewing volcano is now a healthy 100 metres (330 feet) tall.

Kenji Nogami of the Tokyo Institute of Technology, who helped conduct the latest of the coastguard's monthly observations, said volcanic activity is likely to continue for the time being.

"There have not been any significant changes at the volcanic vent of the pyroclastic cone, where eruptions of lava are seen several times a minute," he said in a statement.

"Magma has risen to shallow areas of the vent, and lava flows to the east have continued to stretch out.

"Therefore, I conclude a stable supply of magma is continuing," he said.

The coastguard has asked sailors to stay away from the island.

Japan sits on the Pacific ring of fire, where earthquakes and volcanic eruptions are relatively commonplace.