A new island created by a volcanic eruption off Japan's coast is here to stay -- for now at least, scientists said Tuesday, adding the new landmass could withstand erosion for several years.

Lava that was dramatically vented when an undersea volcano began erupting last month cooled and solidified above the surface of the sea, creating a small island 1,000 kilometres (600 miles) south of Tokyo.

At the time, Japan's coastguard said it was too early to mark a new entry on the national map because it could soon disappear.

But on Tuesday, Japan's meteorological agency said the island looked set to hang around for some time.

"As the volcanic eruption is still continuing, we don't know the fate of the island," said agency official Tomoyuki Kano.

"But it won't disappear in days or weeks, and will probably last for several years... unless a huge volcanic eruption happens and blows it apart," he said.

By early this month the island had grown to more than three-and-a-half times its original size, reaching 0.056 square kilometres (14 acres) by December 4.

"We are still seeing a wisp of smoke and some ash coming from the islet, and occasionally there is lava belching forth, so the islet may grow even bigger," Kano added.

Video footage taken on December 1 showed a thread of white smoke curling out of the top of the islet, with the sea around it turning reddish.

Similar eruptions in the early 1970s and mid-80s created tiny islets in Japan's territory that have since been partially or completely eaten up by the ocean.

While the new island is in uncontested waters, its emergence comes as Tokyo is embroiled in a row with Beijing over the sovereignty of a small archipelago in the East China Sea.

The sudden appearance sparked quips from ministers about the expansion of Japan's territorial waters.