Japan's south-western Kyushu region has had more than 100 aftershocks since it was struck on Tuesday by a powerful earthquake which left at least 13 people dead.

Many others were said to be in "cardiopulmonary arrest" - a term often used in preliminary reports in Japan before a death is officially confirmed.

In searing heat, rescue crews are digging through the rubble at a paper factory and a shopping mall in Kumamoto prefecture, looking for trapped survivors. The mall suffered an explosion in the hour after the quake struck.

Japanese Prime Minister Sanae Takaichi warned it was "truly a race against time" to rescue those trapped. Thousands are in evacuation centres and many are without power.

The Japan Meteorological Agency (JMA) is urging residents to be alert for the possibility of strong aftershocks, public broadcaster NHK reported.

"For the next week or so, especially the next two to three days, we would like people to watch out for potential earthquakes with a maximum seismic intensity of 7," a JMA official said.

Tuesday's quake hit the Kumamoto prefecture on Kyushu island at 16:27 local time (07:27 GMT).

One of the worst-hit buildings was the Aeon Mall in Kashima, where a portion of the second floor collapsed.

Footage of the shopping centre showed white smoke rising as people ran into the car park. Large parts of the centre's exterior were blown off, exposing the steel beams underneath.

Three people remain unaccounted for as rescuers search the rubble.

Aeon Mall president Akio Yoshida confirmed three people were killed in the mall explosion. Some 2,700 people were employed at the mall, officials said.

Yoshida said four people were taken to hospital from the mall, adding that one person had been rescued from a bathroom and another person was in cardiopulmonary arrest.

"We sincerely apologise for this incident," Yoshida said at a press conference on Wednesday.

Other areas affected include a factory in Yatsushiro City where two people were found to have no signs of life and nine others were unaccounted for, NHK reported.

"Although more than 20 hours have passed since the earthquake, there are those still in need of help," Takaichi said on Wednesday.

More than 4,600 personnel were deployed to help with rescue efforts, Takaichi added.

Officials advised rescuers and quake survivors to take precautions against heatstroke in the clean-up work as temperatures hit about 35C (95F).

Defence Minister Shinjirō Koizumi said in a post on X that about 300 air conditioning units would be flown to Kumamoto and distributed in the worst-hit areas.

The JMA measured the earthquake magnitude at 7.1 and the US Geological Survey later placed it at 6.8. Quake magnitude figures are often updated as seismologists obtain and analyse new data.

The quake's shallow epicentre - with a depth of 10 km (six miles) - was about 20km south of Kumamoto city.

Footage aired by NHK showed cracked roads and bridges and several buildings on fire. Train services were suspended and electricity cut off for thousands of homes.

Residents in Kumamoto prefecture awoke on Wednesday to the aftermath of the quake, with images showing debris on the streets and collapsed buildings, including the worship hall of the 19th Century Yatshushiro Shrine.

One resident, Hiroko Ogata, 75, told the BBC she was at the restaurant she runs in Yatsushiro City when the earthquake hit.

"All of a sudden there was a big jolt, and I was surprised and scared... I have lived here for 75 years, but I have never experienced such a big quake. I thought I would die."

American tourist Chantel Mohar was also in Kumamoto with her family when the earthquake struck.

While travelling in the tourist bus, everyone's phones went off with earthquake alerts.

"There was just a lot of rocking and confusion," she told the BBC's Newsday programme.