Rescuers were digging through the rubble of collapsed houses to find survivors after a 7.3-magnitude earthquake struck southern Japan early Saturday, killing at least 32 people and injuring more than 2,000, local media reported.

Two university students were confirmed dead after they were buried under a levelled apartment building in Minamiaso village on the island of Kyushu, broadcaster NHK reported, while many others also died after they became trapped under collapsed houses.

Local police officials in Kumamoto said more people were trapped or buried under rubble. Defence Minister Gen Nakatani told reporters that the ministry would increase the number of troops to be dispatched to the region to 25,000 - with thousands already on site.

The quake hit after at least nine people were killed and 1,100 injured in a magnitude-6.5 earthquake Thursday in the same region - the strongest in five years.

Saturday’s quake occurred at 1:25 am (1625 GMT Friday) with its epicentre in the centre of Kumamoto prefecture at a depth of 12 kilometres, and a series of aftershocks continued to rattle Kumamoto and the neighbouring prefectures, the Meteorological Agency said.

Gen Aoki, director of the earthquake and tsunami department of the meteorological agency, said Thursday’s tremor was now believed to be a “precursor to the main quake” early Saturday.

There was a risk of more aftershocks, as well as mudslides amid expected heavy rain and strong winds overnight in the region, Aoki told a news conference.

Authorities found more damage to houses and buildings Saturday in Mashiki, a town of 33,000, the hardest-hit area of the tremor Thursday evening, 900 kilometres south-west of Tokyo.

TV footage showed flattened houses, collapsing apartment buildings, cracks on roads and a large-scale landslide in Minamiaso village, 20 kilometres to the east of Mashiki.

A 200-metre-long bridge plunged into a valley and the city hall collapsed in Uto, in the south-west of the prefecture and 5 kilometres from the sea. Kumamoto Airport, 15 kilometres north-west of Mashiki, was closed after the ceiling fell, NHK reported.

Meteorological agency officials detected a small eruption at Mount Aso in Kumamoto at around 8:30 am (2330 GMT Friday), but they had not concluded whether the active volcano's most recent eruption was linked to the recent tremors, they said.

Across the prefectures of Kumamoto, Miyazaki and Oita, the quake cut off water supply to about 385,000 households and more than 200,000 households lost electricity, Kyodo News agency reported.

About 170,000 residents were urged to evacuate their homes in the prefectures of Oita and Kumamoto, NHK said.

There were no abnormalities at three nearby nuclear power stations, including the Sendai Nuclear Power Station in Kagoshima prefecture, Chief Cabinet Secretary Yoshihide Suga told a news conference.

The Sendai plant is 130 kilometres south of Kumamoto. Kyushu Electric Power reactivated two nuclear reactors last year, the country's first under updated regulations.

This week’s quakes were the strongest since the earthquake and resulting tsunami in the north-east on March 11, 2011, which prompted Japan to turn off all of its workable reactors in the months that followed.

Those twin natural disasters killed about 18,500 people and triggered the nation’s worst accident at the Fukushima Daiichi Nuclear Power Station.

Earthquake-prone Japan sits at the convergence of four tectonic plates. The country is also home to over 100 active volcanoes.

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