TOKYO: Japan's weather agency on Saturday told thousands of residents near a southern city to prepare for a possible evacuation as it upgraded a volcanic eruption warning.

Officials raised their alert to its second-highest level after picking up increasing seismic activity around the volcano Sakurajima, which sits just off the coast of Kagoshima, a city of more than 600,000 people.

Activity has spiked since early Saturday morning, they said.

The volcano is about 50 kilometres from a nuclear reactor that was switched on this week, as Japan restarted its nuclear power programme following the 2011 Fukushima crisis when a quake-sparked tsunami set off reactor meltdowns at the now-crippled site.

Critics have said the restarted reactor at Sendai was still at risk from natural disasters.

"The possibility for a large-scale eruption has become extremely high for Sakurajima," the agency said, warning residents to exercise "strict caution" and prepare for a possible evacuation.

The warning applies to a part of the island, which is home to more than 4,000 people.

The last major eruption at the 1,117 metre high Sakurajima, a popular tourist attraction, was in 2013 when it spewed ash as far as Kagoshima and sent rocks flying into populated areas, causing damage but no major injuries.

Read more: Japan volcano in spectacular eruption

There are scores of active volcanoes in Japan, which sits on the so-called “Ring of Fire”, where a large proportion of the world's quakes and eruptions are recorded.

Read more: New Japan islet created in volcano eruption

In June, search teams returned to the peak of Mount Ontake in central Nagano prefecture for the first time in eight months to look for the bodies of six climbers still missing after an eruption that killed dozens.

The shock eruption was Japan's deadliest for almost 90 years, leaving an estimated 63 people dead, many of their bodies at least partially entombed in volcanic sludge.