The southern Japanese city of Kumamoto was hit by a 7.3-magnitude quake just days after Kyushu was struck
Over 30 people have been confirmed dead and more than 1,000 injured after two earthquakes hit southern Japan in the last few days.
A 7.3-magnitude earthquake hit southwestern Japan in the early hours of Saturday morning, where at least 25 people were confirmed dead.
The epicentre of the quake was near the city of Kumamoto on the southern island of Kyushu, the same region hit by a 6.2 tremor which killed 10 people barely a day earlier.
The casualty toll is expected to rise as rescue workers dig through collapsed buildings, mud and debris as they look to free dozens of people trapped underneath the rubble.
"Nothing is more important than human life and it's a race against time," said Prime Minister Shinzo Abe. "Daytime today is the big test," he added.
Rescue efforts have been hindered by considerable damage to the region's transport network, with one tunnel caved in, a highway bridge damaged, roads blocked by landslips and train and air services halted.
Heavy rain and strong winds forecast for the next few hours are threatening to further complicate the emergency response.
It is feared that rainfall hitting soil loosened by the quake could set off further mudslides in isolated rural towns, where people in collapsed homes are waiting to be rescued.
Continued tremors are also being felt, with more than 230 aftershocks of at least level one on the Japanese scale reported since Thursday's quake, Japan's meteorological agency said.
One massive landslide has already torn open a mountainside in the Kunamato Prefecture all the way from the top to a highway below.
Almost 200,000 homes are without power across the region, while drinking water systems have failed.
Some 70,000 people have fled their homes.
Hundreds of patients have also been evacuated from a hospital in the southern city of Kumamoto over fears it could collapse.
Japan earthquake: Aerial view of damagehttps://t.co/MrP5h00iEU— Wall Street Journal (@WSJ) 16 April 2016
In a televised address, chief cabinet secretary Yoshihide Suga said: "We are making every effort to respond. Please let's help each other and stay calm."
The number of military personnel sent to the area has been boosted to 20,000, he added. Despite the reports of damage, experts said there was nothing irregular reported at the three nearby nuclear power plants.
Seven hours after the quake a small eruption occurred at Mount Aso in southern Japan, with smoke rising about 100 metres.
Thursday's 6.5-magnitude tremor hit at about 9:26 pm (local time), leaving 10 people dead and injuring around 800 others.
It led to thousands of people spending the night in shelters or under canvas, where they would have been when the latest quake struck.